The concept of one true God who exists eternally in three Persons (Father, Son, and Holy Spirit) is rejected even by some who claim to be Christians. Yet this is taught all through Scripture, in the Old Testament as well as in the New. Consider: “I have not spoken in secret from the beginning; from the time that it was, there am I....” Surely the speaker who has been in existence forever must be God himself. Yet He declares, “the Lord God, and his Spirit, hath sent me” (Isaiah:48:16). Here we have the eternal love of God, who is the Word, declaring that the Father and Holy Spirit sent Him on a mission to this earth. We cannot comprehend the mystery of the Trinity; yet that is no more reason to doubt it than to doubt anything else that we know is real but cannot comprehend.
If God were a single being (as Muslims believe Allah to be and most Jews believe Yahweh is), He would have had to create creatures in order to experience love, fellowship, and communion. The biblical God is love in Himself, manifesting plurality in the Godhead: “The Father loveth the Son...” (John:5:20). God must be one; but He must comprise both singularity and plurality.