At the time when this letter was written, a heresy known eventually as “Gnosticism” was arising and perverting fundamental apostolic teachings. Influenced by Greek philosophers such as Plato, it asserted that matter was evil and spirit was good. Therefore, the false teachers denied Jesus’ true humanity in order to preserve Him from evil. It also claimed a mystical knowledge of truth that was higher even than the Scripture.
Gnosticism had two basic forms. First, some asserted that Jesus’ physical body was not real but only “seemed” to be physical. That explains why John the apostle affirmed the physical reality of Jesus with such wording as “heard”, “seen”, and “touched” (1:1–4; 4:2,3). The second form of this heresy contended that the Christ’s “spirit” descended on the human Jesus at his baptism but left him just before his crucifixion. In denying this, John wrote that the Jesus who was baptized at the beginning of His ministry was the same person who was crucified on the cross (5:6).
The heretical views are destructive to the foundation of true Christian faith, laid by incarnation or “the Word becomes flesh”. Jesus must be a physically real man, because he has suffered and died upon the cross in order to be the substitutionary sacrifice for sin (Heb. 2:14–17). Practically, the Gnostic view of dual humanity with evil material body and good spirit led to the idea that either the body should be treated in an extreme ascetic way, or one does not have to be responsible for his sin in the body because this had no connection with his spirit. This explains why John showed his absolute opposition to the ideas of denying the existence of sin (1:8–10) and of disregarding God’s law (3:4).
The false teachers, coming out there only to expand their personal influence and to gain control over the church, could not show any genuine love for their fellow believers. Therefore, loving each other is a repeatedly emphasized theme in 1 John. The false teachers caused faction in church and even those who remained faithful to apostolic teaching were confused and in urgent need of authoritative clarification of the basics of Christian faith. In this situation, John, with his unquestionable apostolic authority, must reassure and strengthen the believers and battle with the false teachers.
MacArthur, J. Jr: The MacArthur Study Bible. Electronic Ed. Nashville : Word Pub., 1997, c1997, S. 1 Jn 1:1