...He's just saying basic to testing the validity of any teacher as coming truly from God is that teacher's doctrine of Christ. In fact, go back with me to chapter 1 for a moment and John says in verse 1, "That he beheld, and actually touched, the Word of life." That is a term expressing the very deity of Christ, for Christ emanates from God as His living Word. He was in verse 2 with the Father. One with the Father, sharing the same essence with the Father and dwelling in an affable light with the Father before He came to earth. He was manifested to us. The end of verse 3 tells us that our fellowship consequently is with the Father and with His Son Jesus Christ.
John's language then starts out with the fact that Jesus Christ emanates from God as the very living Word of God. There is the written Word of God on the one hand, Scripture. There is the living Word of God, namely the One John says that was from the beginning that we heard, we saw and we touched. That One, that Word of life was the eternal One, the eternal life, verse 2, who was with the Father prior to His incarnation and was then manifested to us in the flesh that we could see and hear and touch. Now that we know Him, and that you know Him, we have fellowship with Him and our fellowship is with the Father and with the Son Jesus Christ. He is therefore the very Word of God incarnate, He is the eternal life who became flesh, He is one with the Father, manifested to us. He is namely the Son in the Trinity who is Jesus Christ. So John starts out with a very definitive Christological statement in his epistle."
John MacArthur, Grace to You, "How to Test the Spirits"