John had just finished warning the church to watch out for false teaching, and continues in this vain. Everyone who goes on ahead and does not abide in the teaching of Christ, does not have God. Whoever abides in the teaching has both the Father and the Son. Obviously new teachings had entered into these churches. Christianity has never been about adapting to the new or to fads. He calls them back to Christ's teaching. Teaching that has authority. To reject these teachings was to reject God himself.
These teaches may have started out as orthodox, but one of things we see in the ministry of the apostles, if we look, is that many of their best and brightest wandered. Demas left Paul, in love with the world (2 Timothy 4:10). These men left John for a fad- and by doing so had forsaken Christ himself. So many in our culture have chosen popularity over the gospel- may we not do the same. John is clear with this heavy warning.
If anyone comes to you and does not bring this teaching, do not receive him into your house or give him any greeting, for whoever greets him takes part in his wicked works.