1 John 4:1-3
Beloved, do not believe every spirit, but test the spirits, whether they are of God; because many false prophets have gone out into the world. By this you know the Spirit of God: Every spirit that confesses that Jesus Christ has come in the flesh is of God, and every spirit that does not confess that Jesus Christ has come in the flesh is not of God. And this is the spirit of the Antichrist, which you have heard was coming, and is now already in the world.
Who do you say Jesus is? God in the flesh! (John 1:1 and 14)
Who is He to you? My personal Saviour and Lord! (Romans 10:9-10)
It's always about Jesus; who He is and what we believe about Him! (Luke 9:20)
False doctrines always argue that Jesus is not God. (Proof of who they are, false teachers.)
John here acknowledges Jesus as the One he had known personally and seen in the flesh, was both divine and human. (Jesus was fully God and fully human.)
Let this mind be in you which was also in Christ Jesus, who, being in the form of God, did not consider it robbery to be equal with God, but made Himself of no reputation, taking the form of a bondservant, and coming in the likeness of men.
For we do not have a High Priest who cannot sympathize with our weaknesses, but was in all points tempted as we are, yet without sin.
1 John 4: 4-6
You are of God, little children, and have overcome them, because He who is in you is greater than he who is in the world. They are of the world. Therefore they speak as of the world, and the world hears them. We are of God. He who knows God hears us; he who is not of God does not hear us. By this we know the spirit of truth and the spirit of error.
This is very clear. If we have the Holy Spirit within us, we have the ability to overcome the world. Only those who know God hear us. If you don't hear or understand, then you are of the world. This is how we know the Spirit of truth compared to the spirit of error. But you have the opportunity to really know God so that you can hear and understand. This is why Christ came into the world - to save us from our sins.
1 John 4:17-19
Love has been perfected among us in this: that we may have boldness in the day of judgment; because as He is, so are we in this world. There is no fear in love; but perfect love casts out fear, because fear involves torment. But he who fears has not been made perfect in love. We love Him because He first loved us.