Ibn Abil-’Izz (d.792H) - rahimahallaah – said, “Knowledge of Usoolud-Deen (the fundamentals of the Religion) is the most noble branch of knowledge, since the excellence of a certain type of knowledge depends upon what it is concerned with, and this is the greater Fiqh (understanding), which is why Imaam Aboo Haneefah (d.150H) - rahmatullaahi ’alayhi - called that which he compiled concerning Usoolud-Deen: “al-Fiqhul-Akbar.” (the Greatest Fiqh). The need of the servants for this knowledge is greater than every other need; and it is the most necessary of all things for them, since there is no life for the hearts, nor any delight, nor any tranquility, except through knowing their Lord, the One to be worshipped, their Creator - with His Names, His Attributes and His Actions, and that He - along with all that - is more beloved to the person than anything else. So man’s striving is with regards to everything that will draw him nearer to Allaah, to the exclusion of the creation.
However, it is impossible for the minds to come to know and understand all that in detail, so the Most Merciful , the Most Majestic - from His mercy, sent Messengers to teach that and call to it; and to give good news to those who accept their Call and to warn those who reject it. The key to their Call and the essence of their message was the servant’s drawing closer to Allaah - the Most Perfect - through His Names, Attributes and Actions, since all that the Messengers were ordered with is built upon this. This then is followed by two great principles: Firstly: Knowing the path that leads to Him - and that is the Sharee’ah which is comprised of His orders and prohibitions. Secondly: That those following the path know what lies in store for them, which is endless bliss. So the people who know Allaah best are the ones who best follow the way to Him; and know best what lies at the end of the way.”
Firmness Upon the Religion:
So, firmness upon the Religion of Allaah, excellence in this world and salvation in the Hereafter is built upon two great matters: “Firstly: Knowledge of Allaah and the beautiful Names and lofty Attributes that befit Him and His Actions - and this necessitates appreciation of His Majesty, honouring Him, fearing Him, being in awe of Him, loving Him, placing one’s hopes in Him, placing reliance upon Him, being pleased with His decree and having patience with what He sends down as regards hardships. Secondly: Knowledge of what He loves and is pleased with, and what He hates and angers Him - whether beliefs saying, or outward or inward actions. So the one who has knowledge of this has to rush to fulfill that which Allaah loves and is pleased with, and to avoid that which He hates and which angers Him.”
Sufyaan Ibn ’Uyaynah (d.197H) - rahimahullaah – said, “There are three types of Scholars: one who knows Allaah and knows Allaah’s commands; and one who knows Allaah, but does not know His commands; and one who knows Allaah’s commands, but does not know Allaah. And the most complete of them is the first - and that is the one who fears Allaah and knows His rulings.”
The Essence of Islaam:
Explaining the essence of Islaam and its main pillar, the Prophet (sallallaahu ’alayhi wa sallam) said, “Islaam is built upon five: Testifying that none has the right to be worshipped except Allaah and that Muhammad is the Messenger of Allaah, establishing the Salaah, paying the Zakaah, making pilgrimage to the House and fasting in Ramadaan.”  In another narration, “Islaam is built upon five: To worship Allaah and to reject anything along with Him... ”  Also in another narration, “Islaam is built upon five: The Tawheed of Allaah... ” 
Thus, “Testifying that none has the right to be worshipped except Allaah,” has the same meaning as, “To worship Allaah and to reject anything along with Him,” which has the same meaning as: “The Tawheed of Allaah,” So, it will be clear to the honourable reader that Tawheed is the essence of Islaam, and it is the starting and ending point for all goodness and excellence.
Linguistically, Tawheed means, “To make something one, or to assert the oneness of something.”  However, what we are concerned with here is the Sharee’ah or technical meaning of Tawheed which is, “To single out Allaah alone for worship.” 
Al-Bayjooree - rahimahullaah – said, “It is to single-out al-Ma’bood (the One to be worshipped - i.e. Allaah) with worship, along with belief and affirmation in the oneness and uniqueness of His Dhaat (Essence), Sifaat (Attributes) and Actions.” 
Shaykh al-Ghunaymaan - hafidhahullaah – said, “It is to single Him out with worship, with love, lowliness and submissiveness to Him, by complying with His commands and submitting to them.” 
The Division of Tawheed:
And Tawheed - with the Salaf and the Scholars of Ahlus-Sunnah wal-Jamaa’ah - has three divisions.
’Allaamah as-Safaareenee (d.1112H) - rahimahullaah – said, “Know that Tawheed has three divisions:- Tawheedur-Ruboobiyyah (the Oneness of Allaah in His Lordship), Tawheedul-Uloohiyyah (to single-out Allaah alone for worship) and Tawheedul-Asmaa‘ was-Sifaat (the uniqueness of Allaah’s Names and Attributes).” 
“And Allaah has gathered these three divisions in His - the Most High – saying,
“Lord of the heavens and the earth and all that is between them, so worship Him alone and be constant and patient in the worship of Him. Do you know of any who is similar to Him?” ” 
“So Tawheedur-Ruboobiyyah implies: A firm and definite belief that Allaah alone is the Creator, the Master and Owner, and the Command is for none but Him.” 
“And Tawheedul- Uloohiyyah is to single-out Allaah alone for all worship and not to worship anything along with Him, whether it be an angel, a Messenger, a Prophet, a pious person, a tree, a stone, the sun, the moon, or other than these.” 
“And Tawheedul-Asmaa‘ was-Sifaat is the uniqueness of Allaah - the Most High - with regards to His Names and His Attributes, by affirming that which Allaah has affirmed for Himself - whether in His Book or by the tongue of His Messenger sallallaahu ’alayhi wa sallam - without tahreef (distorting the Names and Attributes), ta’teel (denial of the Names and Attributes), takyeef (saying how they are), or tamtheel (making any resemblance with the creation).” 
Tawheed in Knowledge and Action:
The above three divisions of Tawheed have been grouped - by some of the Scholars - into two types:- the first type deals with knowing Allaah through His Names, Attributes and Actions (i.e. Tawheedul-Asmaa‘ was-Sifaat and Tawheedur-Ruboobiyyah), and the second deals with actualising and manifesting this Tawheed through singling out Allaah alone for worship (i.e. Tawheedul-Uloohiyyah). The first type of Tawheed is connected with knowledge, whilst the second type is connected with action.
Ibn al-Qayyim (d.756H) - rahimahullaah – said, “As regards the Tawheed which the Messengers called to and which the Books were sent down with, then it is of two types:- Tawheedul-Ma’rifah wal-Ithbaat (the Tawheed of knowledge and affirmation) and Tawheed fit-Talab wal-Qasd (the Tawheed of actions and intentions).
So the first type affirms the reality of the Dhaat (essence) of the Lord - the Most High - along with His Names, His Attributes, His Actions, His speaking in His Books and His speaking to whomsoever He wishes from His servants. It also affirms the all-embracing nature of His Predestination and Pre-Decree and His wisdom. The Qur‘aan has completely clarified this type of Tawheed - as occurs at the start of Soorah Hadeed and Soorah Taa Haa, at the end of Sooratul-Hashr, at the beginning of Soorah Sajdah, at the beginning of Soorah Aali-’Imraan and all of Sooratul-Ikhlaas, and other than these.
The second type: Then it is what is contained in Sooratul-Kaafiroon and contained in His - the Most High’s - saying: “Say: O people of the Book! Come to a word that is just and fair between us and you, that we shall worship none but Allâh and that we shall associate no partner along with Him, and that we shall not take others as lords beside Allâh. Then if they turn away, say: Bear witness that we are Muslims.” 
It is also contained at the beginning of Soorah Tanzeel and at the beginning, middle and end of Sooratul-Mu’min, and at the beginning and end of Sooratul-A’raaf and the greater part of Sooratul-An’aam. Indeed, every Soorah in the Qur‘aan comprises these two types of Tawheed, witnessing to it and calling to it.
Since the Qur‘aan either gives [i] information about Allaah, His Names, His Attributes, His Actions and His Sayings - which is Tawheedul-Ma’rifah wal-Ithbaat (the Tawheed of knowledge and affirmation); or [ii] it is a call to worship Allaah alone without any partner, and a rejection of whatever is worshipped besides Him - and this is Tawheedul-Iraadee at-Talabee (the Tawheed of actions and intentions); or [iii] it is a command to obey Him and to comply with His orders and prohibitions - so this is from the rights of Tawheed and is a completion of it; or [iv] it gives information about the people of Tawheed and how they were treated in this world and how they will be honoured in the Hereafter - and this is the reward for Tawheed; or [v] it gives information about the people of Shirk (associating partners along with Allaah) and the punishment they receive in this world and the torment they shall receive in the Hereafter - so this is the reward for those who abandon Tawheed.”
TAWHEED – ITS IMPORTANCE IN THE QUR‘AAN:
Ibn Abil-’Izz - rahimahullaah – said, “So the Qur‘aan - all of it - is about Tawheed, its rights and its rewards; and about Shirk, its people and their punishments. Thus, “All praise belongs to Allâh, Lord of the worlds.”  is Tawheed (of knowledge). “The Most Merciful, the Bestower of Mercy.” is Tawheed (of knowledge). “The Master of the Day of Judgement.” is also Tawheed (of knowledge). “You alone do we worship and You alone do we seek aid and assistance from.” is Tawheed (of action). “Guide us to the Straight Path.” concerns Tawheed and asking for guidance to the path of the people of Tawheed, (which is), “The Path of those whom You have favoured. Not of those who have earned Your anger, nor of those who have gone astray.” those who have separated themselves from Tawheed.”  So Allaah begins this Soorah by first informing us about Himself - the Most Perfect (i.e. Tawheed of knowledge). Then after having knowledge of Allaah - the Most High - Allaah orders us with the Tawheed of action, “You alone do we worship.”
Likewise just as, “the great Qur‘aan opens with Tawheed it also ends with it. So the Qur‘aan opens with Sooratul-Faatihah, “All praise belongs to Allâh, Lord of the worlds.” and the great Qur‘aan ends with the Soorah, “Say: I seek refuge with the Lord of mankind.” ” 
Likewise, the greatest aayah in the Qur‘aan is Aayatul-Kursee.  Since this great aayah informs us - from start to finish - purely about Allaah’s Names, His Attributes and His Actions. So this further emphasises the importance of the Tawheed of knowledge.
Similarly, Sooratul-Ikhlaas - from start to finish - is concerned with the Tawheed of knowledge, whilst Sooratul-Kaafiroon is connected with the Tawheed of action. Indeed, the Prophet (sallallaahu ’alayhi wa sallam) said about a man who was reciting al-Kaafiroon in the first rak’ah, “This is a servant who believes in his Lord.” Then the man recited al-Ikhlaas in the second, so he (sallallaahu ’alayhi wa sallam) said, “This is a servant who knows his Lord.”  The Prophet sallallâhu ’alayhi wa sallam said, “Say: He is Allaah, the One.” is equivalent to one third of the Qur‘aan. And “Say: O disbelievers...” is equivalent to a quarter of the Qur‘aan. 
What further demonstrates the importance of this Tawheed is the fact that the Prophet (sallallaahu ’alayhi wa sallam), “would begin his day with Tawheed (of knowledge and action), since he recited both Sooratul-Kaafiroon and Sooratul-Ikhlaas in the two rak’aat before Fajr (dawn prayer).  And he would end the night by reciting both of these Soorahs in his witr prayer. ” 
“Indeed, all of the Qur‘aan is about Tawheed.”  And Allaah - the Mighty and Majestic – says, “A book We have sent down, full of blessings, that men may ponder over its messages; and those who possess understanding may take them to heart.” 
TAWHEED – IT’S IMPORTANCE FROM THE SUNNAH:
And what proves the importance of Tawheed from the Sunnah is that the Chosen one - ’alayhis-salaatu was salaam - remained in Makkah thirteen years amongst the disbelievers, calling them to it, and saying to them,
“Say: None has the right to be worshipped except Allâh and thus be successful...” 
The Messenger of Allaah (sallallaahu ’alayhi wa sallam) said, “I have been sent close to the Hour, with the sword, so that none has the right to be worshipped except Allaah alone, without any partner. My sustenance is beneath the shade of my spear. And humiliation and ignominy is for whosoever opposes my command. And whosoever resembles a people is from them.” ” 
Similarly he would send his Companions to various communities ordering them with this Tawheed first - as was the case when he sent Mu’aadh Ibn Jabal to Yemen, saying, “Indeed you are going to a people from the People of the Book, so let the Tawheed of Allaah be the very first thing you call them to...” 
And just as the Prophet (sallallaahu ’alayhi wa sallam) started his Prophethood and teaching with the matter of Tawheed, then likewise, “He (sallallaahu ’alayhi wa sallam) mentioned the matter of Tawheed during his final illness from which he (’alayhis-salaatu was salaam died, when he said, “The curse of Allaah be upon the Jews and the Christians, for they took the graves of their Prophets as places of worship.” ” 
CLEARING A MISCONCEPTION:
Shaykhul-Islaam Ibn Taymiyyah (d.728H) said, “The Tawheed with which the Messengers came with comprises of affirming that Divinity and worship belong to Allaah alone, such that a person witnesses that none has the right to be worshipped except Allaah, and that none is worshipped except Him, nor depended upon other than Him, nor are alliances or enemies made except for Him, nor is an action done except for Him. This affirmation also covers those Names and Attributes which Allaah affirms for Himself, as Allaah - the Most High – says,
“And the Allaah to be worshipped is only one Allaah. None has the right to be worshipped except Him, the Most Merciful, the Bestower of Mercy.” 
And Allaah - the Most High – says, “Do not take two objects of worship. Indeed, Allâh is the only Deity to be worshipped, so fear Him.” .
And He - the Most High – says,
“Whosoever calls upon (i.e. supplicates or prays to) other than Allaah, for which he has no proof, then his reckoning is only with his Lord. Indeed, the disbelievers will not be successful.” 
Allaah - the Most High - also says,
“And ask those of our Messengers whom we sent before you, ‘Did We ever appoint objects of worship besides the Most Merciful?’ ” 
And Allaah informed us about every Prophet from amongst the Prophets that they called the people to the worship of Allaah alone, having no partner, as He said,
“Indeed there is an excellent example for you in Ibraaheem and those with him, when they said to their people, ‘Indeed we are free from you and whatever you worship besides Allaah. We have rejected you, and there has arisen between us and you, hostility and hatred forever - until you believe in Allaah alone.’” 
And Allaah said about the mushriks (pagan idolaters), “When it is said to them, ‘None has the right to be worshipped except Allaah’, they become arrogant. And they say: ‘Are we to abandon those deities that we worship because of a mad poet?’”  And this occurs a lot in the Qur‘aan.
And what is meant by Tawheed is not merely Tawheedur-Ruboobiyyah - the belief that Allaah alone is the Creator - which is what some of the people of Kalaam (speculative theology) and the Soofees think! So they think that if they affirm this type of Tawheed, along with its proofs, then they have affirmed the utmost limits of Tawheed and that if they bear witness to this and become absorbed in it, then they have absorbed themselves in the limits of Tawheed! However, this is not the case. Since even if a person agreed to those attributes which are befitting to the Lord, and declared Him free from everything He should be declared free from and affirmed that He alone is the creator of everything - then still such a person would not be a muwahhid (a person of Tawheed) until along with this, he witnesses that the only Ilaah (i.e. object of worship) is Allaah - (i.e. none has the right to he worshipped except Allaah alone) - affirming that Allaah alone is the Ilaah deserving off all worship, adhering to this worship and associating no partners with Him. Also affirming that this Ilaah (object of worship) is He who is deified and worshipped and who deserves worship and it is not that Ilaah merely with the meaning, “The One who has the power to create and originate.” 
So if an explainer (of the Qur‘aan) explains Ilaah to mean, “The One who has the power to create and originate” and believes that this is the most particular description of Ilaah and affirms this to be the limits of Tawheed - as is done by the people of Kalaam, and it is what they say from Abul-Hasan al-Ash’aree  and his followers - then they do not know the true reality of the Tawheed with which Allaah sent His Messengers, since the Arab mushriks used to agree that Allaah alone is the Creator of everything. Yet despite this, they were still mushriks, as Allaah - the Most High – says,
“Most people do not believe in Allaah except that they commit shirk (i.e. associate others with Him in belief and worship).” 
A group from amongst the Salaf (the first three generations of Muslims) said,
“If you ask them who created the Heavens and the earth, they will say, ‘Allaah’ - yet along with this, they worshipped others besides Him.” 
Allaah - the Most High – said,
“Say: ‘Whose is the earth and all that is in it, if you do indeed know?’ They will say, ‘It belongs to Allaah.’ Say, ‘Will you not then remember?’ Say, ‘Who is the Lord of the heavens and the Lord of the great Throne?’ They will say, ‘Allaah.’ Say, ‘Will you not then fear Him?’”  So not everyone who affirms that Allaah is the Lord of everything and is its Creator, will be a worshipper of Him to the exclusion of everything else - calling upon Him alone, hoping in Him alone, having fear of Him alone, forming allegiance and enmity for Him, obeying His Messengers, ordering what He orders and forbidding what He forbade.” 
Shaykh ’Abdul-Qaadir al-Jeelaanee (d.561H) said, “So it is upon you to fear Allaah - the Mighty and Majestic - and not to fear anyone else except Him. Turn to Allaah - the Mighty and Majestic - for your every need, and rely upon Him alone - the Most High - seeking that which you need from Him alone. Do not rely upon anyone other than Allaah. And Tawheed - all is contained in Tawheed.” 
 Sharhul-’Aqeedatit-Tahaawiyyah (p. 69)
 Fadl ’Ilmus-Salaf (p. 47) of al-Haafidh Ibn Rajab (d.795H)
 Related by ad-Daarimee (1/102) and Aboo Nu’aym in al-Hilyah (7/280), with a Saheeh isnaad.
 Related by al-Bukhaaree (1/49) and Muslim (no. 16) from ’Abdullaah ibn ’Umar - radiyallaahu ’anhu.
 Related by Muslim (no.20)
 Related by Muslim (no.19)
 Lisaanul-’Arab (3/450) of Ibn Mandhoor and also al-Hujjah fee Bayaanil-Mahajjah (1/305) of Abul-Qaasim al-Asbahaanee
 ad-Durarus-Sunniyyah (1/48) of Shaykh ’Abdur-Rahmaan Ibn Hasan
 Jawharut-Tawheed (p. 10)
 Sharh Kitaabut-Tawheed min Saheehil-Bukhaaree (1/38)
 Lawaami’ul-Anwaarul-Bahiyyah (1/128) of as-Safaareenee. For the division of Tawheed into three types, refer to: al-Ibaanah ’an Sharee’atil-Firqatin-Naajiyah (p. 693-694) of Ibn Battah (d.387H); Kitaabut-Tawheed of Ibn Mandah (d.395H) and al-Hujjah fî Bayaanil-Mahajjah (1/85, 1/111-113) of Abul-Qaasim al-Asbahaanee (d.535H)
 Soorah Maryam [19:65]
 Taqreebut-Tadmuriyyah (p. 110) of Shaykh Ibn al-’Uthaymeen
 Taqreebut-Tadmuriyyah (p. 110-111)
 Taqreebut-Tadmuriyyah (p. 112-113)
 Taqreebut-Tadmuriyyah (p. 116-117)
 Soorah aali-’Imraan [3:64]
 Madaarijus-Saalikeen (3/449-450) of Ibn al-Qayyim
 Sooratul-Faatihah [1:1] - and what follows is a completion of this Soorah
 Sharhul-’Aqeedatul-Tahaawiyyah (p. 89-90) of Ibn Abil-’Izz
 Sooratun-Naas [114:1]
 Hukmul-Intimaa‘ (p. 58) of Shaykh Bakr Aboo Zayd - slightly adapted
 Related by Muslim (no. 1768) from ’Ubayy Ibn Ka’b - radiyallaahu ’anhu.
 Refer to Majmoo’ul-Fataawaa (1/54) of Ibn Taymiyyah
 Hasan: Related by at-Tahawee and Ibn Hibbaan. Al-Haafidh Ibn Hajar authenticated it in Ahadeethul-’Aaliyaat (no. 16).
 Saheeh: Related by at-Tabaraanee in al-Mu’jamul-Kabîr (3/203/2) from Ibn ’Umar - radiyallaahu ’anhu. It was authenticated by al-Albaanee in Saheehul-Jaami’ (no. 4405).
 Related by Muslim (no.726)
 Saheeh: Related by an-Nisaa‘ee and al-Haakim, who declared it saheeh.
 at-Tawheed wa Atharahu fee Hayaatil-Muslim (p. 30) of Ahmad Ibn Ibraaheem al-Hareeqee.
 Madaarijus-Saalikeen (3/450)
 Soorah Saad [38:29]
 Related by Ahmad (4/63)
 Saheeh: Related by Ahmad (no. 5114) and Ibn ’Asaakir (19/96/1), from Ibn ’Umar - radiyallaahu ’anhu. It was authenticated by al-Haafidh al-’Iraaqee in Takhreejul-Ihyaa‘ (3/42), and al-Haafidh Ibn Hajr in in Fathul-Baaree (10/222).
 at-Tawheed wa Atharahu fee Hayaatil-Muslim (p. 29)
 Related by al-Bukhaaree (1/13) and Muslim (1/272), from Ibn ’Abbaas - radiyallaahu ’anhu.
 Related by al-Bukhaaree (1/532) and Muslim (5/16)
 at-Tawheed wa Atharahu fee Hayaatil-Muslim (p. 29)
 Soorah aali-’Imraan [3:163]
 Sooratun-Nahl [16:51]
 Sooratul-Mu‘minoon [23:117]
 Sooratuz-Zukhruf [43:45]
 Sooratul-Mumtahinah [60:4]
 Sooratus-Saaffaat [37:35-36]
 This is the saying of the people of Kalaam, such as Aboo Mansoor al-Maatureedee in at-Tawheed (pp. 20-21), and for a similar misconception refer to Fee Dhilaalil-Qur’aan (5/2707) of Sayyid Qutb. Compare this meaning of Ilaah with the correct explanations of great mufassirs - such as Imaam Ibn Jareer at-Tabaree in his Tafseer (20/102) and Ibn Katheer in his Tafseer (3/398) - which is: the one who is who is worshipped and who alone deserves to be worshipped.
 He is Abul-Hasan ’Alee Ibn Ismaa’eel al-Asharee (d.324H) - to whom the Ash’ariyyah ’aqeedah is incorrectly ascribed, since he abandoned this ’aqeedah - and that of the Mu’tazilah previously - for the ’aqeedah of the Salaf - as is mentioned by Ibn Katheer in Tabaqaatush-Shaafi’iyyah and shown by his last book: al-Ibaanah ’an Usoolid-Diyaanah.
 Soorah Yoosuf [12:106]
 This is the saying of lbn ’Abbaas and others - as occurs in Jaami’ul-Bayaan ’an Ta’weelul-Qur‘aan (13/50-51) of at-Tabaree.
 Sooratul-Muminoon [23:84-87]
 Majmoo’ul-Fataawaa (3/97-105) of Ibn Taymiyyah
 Futoohul-Ghayb (p. 176)