Hawzah News Agency (Qom, Iran) - All divinely revealed religions are based on Tawhid, that is, the Oneness of God, and on the worship of this one and only God. The most evident of the principles held in common by all true religions is belief in Tawhid, however much some religious believers may have deviated from this universally held belief. In what follows, we intend to clarify the degrees of Tawhid, with reference to the Holy Quran and the hadiths, and with the application of intellectual reasoning.
 

Oneness of the Essence
The first degree of Tawhid pertains to the Essence (dhat) of God.
We might explain this ‘essential’ Tawhid by saying that the Essence of God is absolutely one and peerless; nothing analogous or similar to Him is conceivable. God’s nature is absolutely simple, non-compound, without any plurality.
 

Imam Ali (as) states, in accordance with these two principles:

‘He is One (wahid) and there is nothing similar to Him among the [existent] things (al-ashya),’ and ‘He—Glorified and Exalted be He!—is one in  meaning [or: spiritual substance] (ma’na); He is not divided into parts by outward existence, by the imagination or by the intellect.’
 

The Sura of Tawhid (al-Ikhlas), the veritable cornerstone of Muslim belief in Divine Unity, alludes to both aspects of this ‘essential’ Tawhid; as regards the first, in the verse: ‘There is none like unto Him,’

and as regards the second, in the verse:

‘Say: He is God, the One.’
In the light of what has been said above, it will be clear that the Christian doctrine of the Trinity—God the Father, the Son and the Holy Spirit—is unacceptable from the point of logic. The inadmissibility of this doctrine has been exposed in certain verses of the Quran, which have been amply commented upon in theological treatises; here, we shall limit ourselves to the following, altogether sufficient, argument.
 

The Trinity, in the sense of three gods, must mean one of two things: (a) either it means that each of the three gods possesses a distinct ontological personality, along with all attributes of divinity—in which case ‘essential’ Tawhid is contradicted in respect of its first meaning, that is, He has no peer or like; (b) or else it means that these three gods partake of a single ontological personality, such that each is a part of the whole—in which case such an entity would perforce be compound, thus contradicting the second meaning of ‘essential’ Tawhid, namely, that God is absolutely simple and not composed of ‘parts’.

Reference:

 Ayatollah Jafar Sobhani, Doctrines of Shii Islam, A Compendium of Imami Beliefs and Practices, Translated and Edited by Reza Shah-Kazemi, published by I.B.Tauris Publishers, London • New york  2003