In his first letter, John combats different heresies to make sure it is clear that Jesus was fully God and fully man. Belief in both parts is the only way that a person can truly know eternal life.
1 John 1:1-4
1 What was from the beginning, what we have heard, what we have seen with our eyes, what we have looked at and touched with our hands, concerning the Word of Life—
2 and the life was manifested, and we have seen and testify and proclaim to you the eternal life, which was with the Father and was manifested to us—
3 what we have seen and heard we proclaim to you also, so that you too may have fellowship with us; and indeed our fellowship is with the Father, and with His Son Jesus Christ.
4 These things we write, so that our joy may be made complete.