The Bible teaches that God is One and also that there are three Persons in this One God.
Since numbers belong to the material world, and since God is Spirit, our finite mind cannot grasp this truth fully, even as a small cup cannot contain the water in an ocean.
A dog cannot understand multiplication - how three ones when multiplied can still be one: 1x1x1=1. Neither can we understand how God can be Three Persons and yet One God. A dog can understand only another dog. He cannot understand a man fully. In the same way, a god that could be explained and understood by our human reason would only be another man just like ourselves. The very fact that the God of the Bible transcends our reason is the clearest evidence that this is indeed the truth.
The truth of the Trinity is clear from the first verse of the Bible, where the word for `God' is plural in the Hebrew - `Elohim'. We also see it in the use of the words `Us' and `Our' in Gen. 1:26. The light is focused more clearly at the baptism of Jesus, where the Father (voice from heaven), the Son (Jesus Christ) and the Holy Spirit (in the form of a dove) are all present (Matt. 3:16,17).
Those who say that Jesus Himself is Father, Son and Holy Spirit cannot explain how He could then have done His Father's will on earth, while denying His own will (Jn.6:38). Unitarians who believe that God is just One Person, and who therefore baptize in the Name of "Jesus only" are therefore denying that Jesus came as a Man.
The Bible says that the one who has the right teaching, has both the Father and the Son, and that those who deny either the Father or the Son have the spirit of the Antichrist (2 Jn.9; 1 Jn.2:22).
In Christian baptism, Jesus commanded us specifically that we were to baptize in the threefold Name of the Father, Son and Holy Spirit (Matt. 28:19), the Son being identified as the Lord Jesus Christ (Acts 2:38).
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