“Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost.” (Matthew 28:19)
The foundational plank of Israel’s worship was Deuteronomy 6:4—“Hear, O Israel: The LORD our God is one LORD.” Even the great apostle James acknowledged, “Thou believest that there is one God; thou doest well: the devils also believe, and tremble” (James 2:19).
Some have suggested that the Old Testament does not teach the Trinity and that the New Testament is making a “god” out of Jesus to foster the new religion. Nothing could be further from the truth. Both sections present the triune God.
The Father is named in Malachi 2:10: “Have we not all one father? hath not one God created us?” Jesus Himself insisted that we pray to the Father. “After this manner therefore pray ye: Our Father which art in heaven, Hallowed be thy name” (Matthew 6:9).
The Son is clearly declared in both Testaments. “The LORD hath said unto me, Thou art my Son; this day have I begotten thee” (Psalm 2:7). Jesus said, “I and the Father are one” (John 10:30), and the apostle Paul insisted that the Lord Jesus was “all the fulness of the Godhead bodily” (Colossians 2:9).
The Holy Spirit is hardly a stranger to both Testaments. King David knew that “the Spirit of the LORD spake by me, and his word was in my tongue” (2 Samuel 23:2). And as the Lord Jesus was preparing to go back to the Father, He promised that “the Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you” (John 14:26).
This much is clear: There is one God, who is manifested to us in three Persons. HMM III