What is Tawheed, and what are its three types?

Belief in the Oneness of Allah, Tawheed, has three types: Tawheed al-Uloohiyah, the Oneness of Divinity; Tawheed ar-Ruboobiyah, the Oneness of Allah's Lordship; and Tawheed al-Asma was-Sifat, the Oneness of Allah's Names and Attributes.

Oneness of Divinity or State, Tawheed al-Uloohiyah, is to believe that Allah is the only God who has the right of Uloohiyah, or Divinity, over all His creatures. There is absolutely no creature that has a Divine attribute, and He stands in need of none of His creatures. Allah says:

"Say, the truth is that Allah is One. Allah is Besought of all, needing none, He neither begets nor was He begotten. Nor is there anyone who is comparable to Him." [Qur'an, al-Ikhlas 112:1-4]

He is the only God Who deserves to be worshipped, hence no devotional rite should be devoted to other than Allah. Believing otherwise constitutes major shirk, or polytheism.

The belief of Oneness of Allah's Lordship, Tawheed ar-Ruboobiyah, is the belief that Allah alone is the Rubb, the Lord, of everything: the Creator, the Provider, the Proprietor, the One in Whose hand is the disposal of all affairs. He has the power over all things. He gives life, causes death, grants honour, victory and defeat.

The term Rubb is derived from the verb "to rear" or "to raise". Since Allah is the One Who provides man with all his needs from His bounty, He rears His creatures and nourishes them with His favours and bounty. In return, man should express his gratitude to Allah by worshipping Him alone, ascribing no partner to Him.

Tawheed ar-Ruboobiyah is the central creed, yet upholding it without upholding the other two types of Tawheed is not enough to save one from Hell. The pagans at the time of the Prophet ﷺ acknowledged that Allah is the Creator of the heavens and the earth, and the Provider, yet this would not benefit them or save them from the Fire on the Day of Resurrection, as Allah mentions in the Qur'an:

"If you should ask them who created Heaven and Earth, they would say: 'The Powerful, the Aware has created them'" [Qur'an, az-Zukhruf 43:9]

The belief of Oneness of Allah's Names and Attributes, Tawheed al-Asma was-Sifat, is to believe that Allah enjoys unique Divine names and attributes with which Allah has named and qualified Himself, and those with which His Messenger ﷺ named and qualified Him, without comparing Him with His creatures, and without suspending or distorting the meaning of His names and attributes:

"Lord of the heavens and the earth, and all that is between them, so worship Him (Alone) and be constant and patient in His worship. Do you know of any who is similar to Him?" [Qur'an, Maryam 19:65]

Tawheed ar-Ruboobiyah entails believing in Tawheed al-Uloohiyah, that is, to believe in Allah as the Rubb, as described above, and to acknowledge by necessity that no one deserves to be worshipped except Allah.