Beloved, believe not every spirit, but try the spirits whether they are of God: because many false prophets are gone out into the world. Hereby know ye the Spirit of God: Every spirit that confesseth that Jesus Christ is come in the flesh is of God: And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world. ~ 1 John 4:1-3

The confession that Jesus Christ has come in the flesh speaks to the deity of Christ. In chapter one, John refers to Jesus as the word of life and the eternal life which was with the Father from the beginning, and was manifested to us.

That which was from the beginning, which we have heard, which we have seen with our eyes, which we have looked upon, and our hands have handled, of the Word of life; (For the life was manifested, and we have seen it, and bear witness, and shew unto you that eternal life, which was with the Father, and was manifested unto us;) That which we have seen and heard declare we unto you, that ye also may have fellowship with us: and truly our fellowship is with the Father, and with his Son Jesus Christ. ~ 1 John 1:1-3

John refers to the deity of Christ at the very outset of his epistle by referring to Jesus as the word of life and eternal life. John testifies that Jesus, the Son of God, is the eternal life which was with the Father from the beginning.

John uses similar language in his gospel.

In the beginning was the Word, and the Word was with God, and the Word was God. The same was in the beginning with God. All things were made by him; and without him was not any thing made that was made. In him was life; and the life was the light of men… And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth. ~ John 1:1-4, 14

John is heavy on the deity of Christ, such much so, that John declares that fellowship with God is contingent Jesus being divine: That which we have seen and heard declare we unto you, that ye also may have fellowship with us: and truly our fellowship is with the Father, and with his Son Jesus Christ.

Fellowship with the Father and his Son Jesus Christ is fellowship with the Father,  and the One whom John claims is eternal life. In chapter 2, John addresses fathers, young men, and little children (most likely these are references to spiritual growth). To the fathers, he says, I write unto you, fathers, because ye have known him that is from the beginning (2:13) and,  I have written unto you, fathers, because ye have known him that is from the beginning (2:14). 

At the close of his epistle, John declares Jesus is the true God!

And we know that the Son of God is come, and hath given us an understanding, that we may know him that is true, and we are in him that is true, even in his Son Jesus Christ. this is the true God, and eternal life. ~ 1 John 5:20

Now, with regards to John’s statement concerning the confession that Jesus Christ has come in the flesh: Jesus was God revealed in the flesh. Every spirit that does not confess God, in the person of Jesus Christ, was manifested in the flesh is the spirit of anti-Christ.

The Spirit of God exalts Christ. The spirit of anti-Christ makes less of Christ. One does not have true fellowship with God if he does not have fellowship with that eternal life which was with the Father from the beginning and was manifested in the flesh.

To claim otherwise is to be a liar and a deceiver.

Who is a liar but he that denieth that Jesus is the Christ? He is antichrist, that denieth the Father and the Son. Whosoever denieth the Son, the same hath not the Father: he that acknowledgeth the Son hath the Father also. ~ 1 John 2:22-23

(For the life was manifested, and we have seen it, and bear witness, and shew unto you that eternal life, which was with the Father, and was manifested unto us;) That which we have seen and heard declare we unto you, that ye also may have fellowship with us: and truly our fellowship is with the Father, and with his Son Jesus Christ. ~ 1 John 1:2-3