This is the first of several times when John challenges the claims of the Gnostics. These teachers had separated themselves from the fellowship of the apostles, so they were living in spiritual darkness. They claimed to have fellowship with God, but they did not show God’s character, which is light.
Jesus warned the Jewish leaders of his day not to let the light they believed they had become darkness (Matthew 6:23). Their religious beliefs kept them from receiving the true spiritual light that Christ offered. In the same way, the Gnostic teachers believed they were enlightened, but their teachings actually led them into darkness.
They claimed to receive spiritual experiences from God. Yet they rejected fellowship with the apostles, who had truly seen God in the flesh in the person of Jesus Christ.