The truth about the Trinity is a dominant focus of each of the three Christian creeds.
Closely related and equally important are the creeds’ declarations about the Son, Who took on humanity to reconcile men to the Father.
We can see what a heated issue this was already in Jesus’ ministry, when the Jews were ready to stone Him for declaring, “I and the Father are one” (John 10:30-33).
In his epistles, John makes this the decisive issue in distinguishing between truth and falsehood:
“By this you know the Spirit of God: every spirit that confesses that Jesus Christ has come in the flesh is from God; and every spirit that does not confess Jesus is not from God; this is the spirit of the antichrist ” (1 John 4:2-3).
“For many deceivers have gone out into the world, those who do not acknowledge Jesus Christ as coming in the flesh” (2 John 7).
The earliest Christian “creeds” (declarations of faith) appear already in the New Testament.
One of the most common and significant confessions was “Jesus is Lord” (Rom 10:9; 1 Cor 12:3).
With these three words “early Christians acknowledged that the Nazarene was to be spoken of in the same terms as Yahweh of the Old Testament” (B. Demarest, “Creeds,” New Dictionary of Theology, 179).
At every Christian baptism the new believer is asked to acknowledge that Jesus is the Son of God Who became a man in this world to reconcile us to God the Father by His iife. death, and resurrection.
(Spotlight 2, Lesson 4 in Doctrine 101: Learning about God)