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. – 2 John 1:10-11 ESV
John has written about deceivers, specifically those who claim that Jesus Christ has not come in the flesh. John calls them antichrists, people who are advocating a different Christ than is presented in the New Testament. Some antichrists deny the incarnation, that God took on human flesh in the person of Jesus. Others deny that Jesus is God. And still others will deny that Jesus is the only way to the Father. But what each of these share is that they are antichrists.
And John, in his typically blunt fashion tells us to have nothing to do with these antichrists; do not offer them hospitality, do not welcome them, and do not offer them God speed. For in doing so we encourage them and become participants in what they are doing.
Many of our doctrinal differences are not overly significant. But what we believe about the person of Christ is essential. Jesus is fully God and fully man. He is of one essence with God the Father and God the Holy Spirit, but is a distinct person, as is the Father and the Holy Spirit. The one who would teach otherwise is antichrist. Don’t fall under their influence or encourage them in any way.