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 Spirit. The Athanasian Creed states:
"There is one person of the Father, another of the Son, and another of the Holy Spirit. But the Godhead of the Father, of the Son, and of the Holy Spirit, is all one; the Glory equal, the Majesty co-eternal The Father is God, the Son is God, and the Holy Spirit is God. And yet they are not three Gods, but one God... For like as we are compelled by the Christian verity to acknowledge every person by himself to be God and Lord, so are we forbidden by the Catholic religion to say there be three Gods, or three Lords."
This is obviously self-contradictory. It is like saying, one plus one plus one is three, yet it is one. lf 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."
Strangely enough, 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 O 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 God, as is evident from the following saying:
"Thou shalt worship the Lord thy God and Him only shalt thou serve."
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 A.C., 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 Encyclopaedia (bearing the Nihil Obstat and lmprimatur, indicating official approval) 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:
"lt is difficult, in the second half of the 20th century to offer a clear, objective, and straightforward account of the revelation, doctrinal evolution, and theological elaboration of the mystery of the Trinity. Trinitarian discussion, Roman Catholic as well as other, presents a
somewhat unsteady silhouette. Two things have been happened. There is the recognition on the part of exegetes and Biblical theologians, including a constantly growing number of Roman Catholics, that one should not speak of Trinitarianism in the new Testament without serious
qualification. There is also the closely parallel recognition on the part of historians of dogma and
systematic theologians that when one does speak of an unqualified Trinitarianism, one has moved from the period of Christian origins to, say, the last quarter of the fourth century. It was only then that what might be called the definitive Trinitarian dogma 'one God in three persons' became thoroughly assimilated into Christian life and thought."
A little later the same Encyclopaedia says even more emphatically:
"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.'"
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 has become a part of the Christian faith towards the end of the fourth century.
Rationally considered also the dogma of the Trinity is untenable. It is not just beyond reason, it is repugnant to reason. As we said earlier, the belief in three Divine Persons is incompatible with the Oneness of God. If there are three distinct and separate persons, then there must be three distinct and separate substances, for every person is inseparable from its own substance. Now if the Father is God, the Son is God, and the Holy Spirit is God, then unless the Father, the Son, and the Holy Spirit are three distinct Northings, they must be three distinct substances, and consequently three distinct Gods. Furthermore the three Divine Persons are either infinite or finite. If infinite, then there are three distinct Infinites, three Omnipotents, three Eternals, and so three Gods. If they are finite, then we are led to the absurdity of conceiving of an Infinite Being having three finite modes of subsisting or of three persons who are separately finite making up an infinite conjunctly. The fact is that if the Three Persons are finite, then neither the Father, nor the Son, nor yet the Holy Spirit is God.
The doctrine of the Trinity was developed as a consequence of the deification of two creatures, Jesus Christ and the mysterious Holy Spirit, and their association with God as partners in His Godhead. As explained in Christian literature it amounts to the separate personification of three attributes of God. Whether considered from historical view point or otherwise, it is a regression from rational theology to mythology. For, at the root of all mythologies lies the irrational tendency of the human mind to deify great men and personify non-personal forces and
attributes and to present them as Divine Persons.
Islam preaches the plain and simple Unity of God. It presents a conception of God that is free from anthropomorphic of mythological fancies. It affirms the uniqueness of God and says that He has no partner in His Godhead. He is One in person and one in substance, the two being indistinguishable. 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:
(Say: He is God, the One; God, the eternally Besought of all. He begetteth not nor was begotten. And there is none comparable unto Him)
(Qur'an 112: 1-4)
(Your God is One God; there is no god save Him, the All-Loving, the All-Merciful. Lo! in the creation of heavens and the earth, and the difference of night and day, and the ships which run upon the sea with that which is of use to men, and the water which God sendeth down from the sky, thereby reviving the earth after its death, and dispersing all kinds of beasts therein, and in the ordinance of the winds, and the clouds obedient between heaven and earth, are signs (of God 's Unity and Sovereignty) for people who have sense.)
(God, there is no god save Him, the Alive, the Eternal. Neither slumber nor sleep overtaketh Him. Unto Him, beiongeth whatsoever is in the heavens and whatsoever is in the earth. Who is he that intercedeth with Him save by His Leave? He knoweth that which is in front of them and that which is behind them, while they encompass nothing of His Knowledge save what He will, and His Throne includeth the heavens and the earth, and He is never weary of preserving
them. He is the Sublime, the Absolute.) (2:255)
 Rev J. F. De Groot, Catholic Teaching, p. 101.
 The New Catholic Encyclopaedia (1967), art. "The Holy Trinity" Volume 14, p.295.
 The New Catholic Encyclopaedia (1967) , art." The Holy Trinity", Volume 14, p. 299.