All passages of Scripture must be understood in context, but some by their nature require more contextualization than others. First John is one such book. Without an understanding of the errors it was written to refute, the necessary result will always be more error, or at the very least, more confusion.
Those believers who had not gone with them were overcomers 4:4), but they were overcomers who had been unsettled by the battle and who were greatly in need of encouragement. This is the pastoral encouragement that John provides in this letter.
What were the characteristics of the lies they had to deal with? From the internal evidence of this letter, and from the external evidence we have about Ephesus in the first century, we can piece together a pretty good understanding of the heresy John was attacking. The false teachers had both a doctrinal problem (2:26) and an ethical/moral problem (3:7). The doctrinal problem was that they denied the incarnation of Jesus. The ethical problem was that they claimed to be able to be “in the light” while taking some kind of weak view of their sinfulness and sins. They were somehow above it all, more mature than to have to deal with boring ideas of sin.
As it happens, we know a good deal about this brand of heresy in Ephesus at this time. The leader of the opposition against the apostle John in Ephesus was a man named Cerinthus. He was a leader of an early Gnostic group, and the Gnosticism was characterized by two great features—the necessary impurity of matter and the supremacy of knowledge. The first led them to deny the incarnation, which is in effect the materialization of the eternal one. Their arrogance and pride over their “inside knowledge” led them to their lovelessness and their lawlessness. Hence, we see John attack the heresy of Cerinthus at every key point. Christ is the very Son of God. We must walk in love. We must keep God’s commandments.
The first four verses begin the epistle without preliminary introduction. First we have the relative clauses — John begins with that which was from the beginning. But the eternal came here, and was seen, heard, and touched. John himself had leaned back on the Lord at the Last Supper. Our message — the word of life — is the word about the Word. Put another way, the gospel is the preaching of Christ.
The second thing is the importance of testifying and preaching. The first verb testify refers to the authority of the witness. The second refers to the authority of one commissioned.
Then we have intertwined fellowship. The gospel is declared in such a way as it creates fellowship between those who speak and those who hear, provided that those who speak are in fellowship with both the Father and the Son. The fellowship John has in mind here is not simply human sociability. This is a religious fellowship based upon transcendent realities. The reality in question here is the nature and meaning of that transcendent reality that came to live with us, as witnessed by the incarnation of Christ.
And last is the fullness of joy. John writes so that their joy would be full. This is only possible if their joy is built on the right foundation. That foundation must be the truth — truth about God, truth in God, true fellowship with God, and consequently, true fellowship with the others who are faithful.
As we consider these issues, we must come to understand that theological subtleties matter. The false teachers here did not deny that Jesus was the Christ — Cerinthus taught that Jesus was the Christ from right after his baptism until right before his death. In contrast, John insists that Jesus, the incarnate God, passed through the water and the blood (5:6).