Why Do Christians Believe in the Trinity?

A Reasonable Answer to Unitarian Criticism of Trinitarian Doctrine

Trinity Defined
Muslims and Unitarians criticize the idea of the Trinity as illogical; they say it violates the logical Law of Non-contradiction: "A" cannot be "non-A" at the same time and in the same respect (or aspect). They claim that Christians say that three gods, the Father, the Son, and the Spirit are one God. And if that were what the doctrine said, they would be correct in rejecting it as illogical.

Muslims misunderstand the doctrine, perhaps, because many Christians misunderstand it and are unable to correctly state it. Following Dr. Walter Martin, the doctrine can be stated thus: The one God eternally exists as the Father, the Son, and the Spirit. Thus, God is one as to his being, essence or nature, but three as to Persons.

Oneness in Diversity
Now in the Old Testament, God’s oneness is expressed in the word echod, which is a unity that contains diversity. For example, this word occurs in the Jewish affirmation of faith of Deuteronomy 6:4, the Shema: “Hear, O Israel, the LORD our God, is one LORD.” Echod also occurs in Genesis 2:24: “For this reason a man will leave his father and mother and be united to his wife, and they will become one flesh.”

We often say that marriage is an image of the relationship of God and man, which is based on the biblical metaphor of Christ as the Bridegroom and the collective Body of Christ, the Church, as his Bride. (The individual believer, male or female, is not a “bride” of Christ, and “bridal mysticism” along this line has led to false doctrine and demonization.) But the profoundest symbolism of marriage is in its imaging of the loving unity of the Persons of the Holy Trinity.

Threeness
The threeness of God makes the Apostle John's statement in 1 John 4:8 and again in verse 16 that “God is love” possible. The three persons have been bound together in the unity of perfect love forever. God's love is the structural glue that unites the Trinity and forms the basis for God's fatherly love and of Christ’s husbandly love for the church. Love as I—Thou relationship is only possible among two or more persons. Thus for God to be love, the One God must have more than one Person within his unitary Being.

Consequences of Unitarianism
Muslims deny that God is Trinity and affirm that He is unitary. Logically enough, then, they deny that God deigns to call himself the Father of any human being. Their God is a monad, our Judge, exalted, utterly transcendent. And that explains why he is not seen as a loving father. Love as I—Thou relationship is not possible within a monistic God and therefore a fatherly relationship with his creatures is not to be expected. This is the logical result of any form of unitarianism and should be pointed out to Unitarian-Universalists and to Jews, who lack the logical rigor of Muslims in this respect.