“It is what we know as the doctrine of the Trinity. In telling us who and what Jesus Christ really is, John thus introduces us to the doctrine of the Trinity. If we were told nothing about the Trinity except what we are told in a single verse, it would yet lie before us in its whole principle. There is no other key which will unlock the mystery of the eternal Being of the Word as here described to us. We are but expressing John’s meaning, then – in other words, but nevertheless nothing but his meaning – when we declare that Jesus Christ is the Second Person of the Adorable Trinity. This is, in brief, what John teaches us in the first verse of his Gospel.” B.B. Warfield, The Glory of the Word, Faith & Life.
In the beginning was the Word, and the Word was with God, and the Word was God.
John 1:1 (ESV)