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.
To our 21st century eyes, this makes us sit up straight in our seat. "Wait, we're not supposed to welcome them in?" A few things need to be said in way of background. First, the church or churches John is writing to most likely met in houses. So, to welcome someone into your house was not saying not to have them over for dinner, but to welcome to them into the body as a brother. IT assumed them to be a true Christian. Second, the first century gathering was not a place to reach unbelievers but a place to equip believers. In the first century, death would await the Christians who gathered, so these Christian worship gatherings were not a place to invite their Pagans neighbors who would have had them arrested and/or killed.
So, to invite someone into your house was to welcome them fully as a trusted member of the body. But, those who do not abide the teaching of Christ should not be given that welcome. They are not brothers, they are enemies of the teaching of Christ, and could possibly sell these First Century Christians into the hands of the Romans. Also, to fully embrace them as a brother or sister would mean to fully embrace their false teachings about Jesus which is dangerous for them, but false teaching is also to the congregation. As Paul writes, sound doctrine saves both the believer and their hearers (1 Timothy 4:16). It was for the sake of love and the sake of the truth that they did not welcome false teachers into the body.
John loved these people, as is clear as he closes his letter to them. He has so much he would like to say, but he can't. His joy would not be complete until they see each other face to face. As the last two verses read,
Though I have much to write to you, I would rather not use paper and ink. Instead I hope to come to you and talk face to face, so that our joy may be complete. The children of your elect sister greet you.
Love and truth bind us together and create joy-filled community. Of all John could have said in his epistle, their are certain things he just desired to say to their face! Nothing can replace the local church in John's heart. Even if he could have live-streamed the gathering, it would not be a proper replacement for being bodily present in the gathering! His love for the truth was a desire for their joy. Deception abounds which seeks to kill our joy, but the local church exists that we our hearts may be on fire with love for God, and our minds might be captivated with the wonderful truth of Himself. The local church is not an option for the Christian, it was not seen as an option for John, but it existed that we may love and treasure God more- for our joy and His glory.