"Say: He is Allah, the One; Allah, the Eternal, Absolute; He begetteth not, nor is He begotten, and there is none like unto Him." [Quran 112:1-4]  

"The doctrine of the Trinity is that there are three separate and distinct Divine Persons in Godhead-God the Father, God the Son, and God the Holy Ghost. The Athanasian Creed states:

"There is one person of the Father, another of the Son, and another of the Holy Ghost. But the Godhead of the Father, of the Son, and of the Holy Ghost, is all one; the Glory equal, the Majesty co-eternal... The Father is God, the Son is God, and the Holy Ghost is God. And yet they are not three Gods, but one God...."

This is a self-contradictory. It is like saying one plus one plus one is three, yet it is one. If there are three separate and distinct Divine Persons and each is God, then there must be three Gods. The Christian Church recognizes the impossibility of harmonizing the belief in three Divine Persons with the oneness of God, and hence declares the doctrine of the Trinity to be a mystery, in which a person must have blind faith. This is what the Rev. J.F. De Groot writes in his book Catholic Teaching:

"The Most Holy Trinity is a mystery in the strictest sense of the word. For reason alone cannot prove the existence of a Triune God, Revelation teaches it. And even after the existence of the mystery has been revealed to us, it remains impossible for the human intellect to grasp how the Three Persons have but one Divine Nature." [l]

Jesus Christ himself never even mentioned the Trinity. He knew or said nothing at all about there being three Divine Persons in Godhead. His conception of God was in no way different from that of the earlier Israelite prophets, who had always preached the Unity of God and never the Trinity. Jesus merely echoed the earlier prophets when he said:

"The first of all the commandments is, Hear 0 Israel; The Lord our God is one Lord; and that thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind, and with all thy strength". (Mark 12:29,30)

He believed in One Divine Person, One God, as is evident from the following saying:

"Thou shalt worship the Lord thy God and Him only shalt thou serve". (Matthew 4:10).

The doctrine of the Trinity was coined by the Christians about three hundred years after Jesus. The four Canonical Gospels, written between 70 and 115 C.E., contain no reference to the Trinity. Even St. Paul, who imported many foreign ideas into Christianity, knew nothing of the Triune God. The New Catholic Encyclopedia admits that the doctrine of the Trinity was unknown to the early Christians and that it was formulated in the last quarter of the fourth century (see The New Catholic Encyclopedia (1967), art, "The Holy Trinity", Volume 14, P.295...).

The same Encyclopedia says:

"The formulation ‘one God in three persons' was not solidly established into Christian life and its profession of faith prior to the end of the 4th century. But it is precisely this formulation that has first claim to the title the Trinitarian dogma. Among the Apostolic, Fathers, there had been nothing even remotely approaching such a mentality or perspective."[2]

So the doctrine of the Trinity was not taught by Jesus Christ, it is nowhere found in the Bible (both the Old and the new Testaments), it was completely foreign to the mentality and perspective of the early Christians; it became a part of the Christian faith towards the end of the fourth century.

Quran preaches the plain and simple Unity of God. It presents a conception of God, which is free from anthropomorphic or mythological fancies. It affirms the uniqueness of God and says that He has no partners in his Godhead. He is one in person and one in substance-the two being in-distinguishable. He is the Self-sufficient One, on whom all depend and Who depends not on any one. He is the Creator and Nourisher of all, the All-Good, the All-Mighty, the All-Knowing, the All-Loving, the All-Merciful, the Eternal and the Infinite. He neither begets, nor is begotten. Nothing can come out of Him and become His equal and partner in Godhead." [3]

"Say: He is Allah, the One; Allah, the Eternal, Absolute; He begetteth not, nor is He begotten, and there is none like unto Him." [Quran 112:1-4]

References:

[1] Rev. J.F.De Groot, Catholic Teaching, P.101

[2] The New Catholic Encyclpaedia (1967), art "The Holy Trinity", Volume 14, P.299

Source:

[3] Islam and Christianity by Mrs. Ulfat Aziz-us-Samad