The word tawhid, infinitive of the second form of the Arabic verb w-h-d, literally means making one or asserting oneness. Derivations include wahhada means to unite, unify, connect, join, profess; wahdah means oneness, singleness, al-wahid means the One and al-ahad means the singular without number. It is applied theologically to the Oneness (wahdaniya, tawahhud) of God in all its meanings. It is the first and basic brick to believe in Islam, i.e. faith in the Unity of God. In Koran, God is described 13 times as Sole Divinity (ilah wahid) and 29 times as No god other than He (la ilaha illa huwa). The best known confession of tawhid is contained in la ilaha ill-Allah, which is made up of four words: la (no), ilah (that which is worshipped), illa (except) and Allah (the proper name of Divine Being), thus it means "there is no god but Allah." The Unity of God implies that God is One in His person (Dhat) and One in His attributes (Sifat).
The preamble of the Shi'ite Ismaili Muslims Constitution begins with the declaration: "The Sh'ia Imami Ismaili Muslims affirm the Shahadah, La ilaha illallah, Muhammadur Rasulullah." For Ismaili Muslims, as for other Muslims around the world, the concept of tawhid, the unity of God, is the cornerstone of the faith and forms the cardinal principle of Islam.
God is One, Unique, beyond comparisons and descriptions, beyond human weakness and limitations, beyond human thoughts and imaginings of Him. Islam is uncompromisingly monotheistic in this aspect; the greatest sin in Islam is shirk, which means to attribute partners in God. In Islam, God is One, Unique, beyond any partners, consorts, off-springs or equals, to the extent that He does not even have an opposite.
The doctrine of tawhid is beautifully expressed in the Sura Ikhlas of the Koran. The word ikhlas is derived from khalasa means sincerity, purity, freedom, clear or unmixed, and thus, ikhas carries the connotation of purification, because this chapter accomplishes the purification of the concept of God from all human misconception. It reads: "Say, He, Allah, is One! Allah is Absolute, Independent. He did not beget, nor was he begotten. And there is none like unto Him." The word ahad is translated by different commentators as One and the Unique.
Imam Ali bin Abu Talib said, "He (God) is such a Being that He had no beginning and nothing existed before Him, and He has no end and nothing can be imagined to exist after Him." Imam Jafar Sadik said, "What God is man cannot think, and what man thinks, God is not. Yet man lives by God and God is nearer to him than himself."
The Absolute transcendence of Godhead (al-Mubdi) is counter balanced with the immanence of mediation. It is through the mediation of the Prophet or Imam, each in his time, a vision of the Absolute Reality can be attained. Through rational faculties are incapable of comprehending the Truth, there is a deeper and more esoteric dimension of human consciousness, which is capable of experiencing the Truth. The Prophet or an Imam, as the bearer of the Divine Light, awakens that esoteric dimension in man. It is through their exoteric and esoteric teachings that they guide and prepare the believers to achieve that experience.