Don't Even Greet False Teachers

2Jn 1:10  If anyone comes to you and does not bring this teaching, do not receive him into your house, and do not give him a greeting; 11  for the one who gives him a greeting participates in his evil deeds.

"greet":

chairō
khah'ee-ro

A primary verb; to be full of “cheer”, that is, calmly happy or well off; impersonal especially as a salutation (on meeting or parting), be well: - farewell, be glad, God speed, greeting, hail, joy (-fully), rejoice.

The Christian is not to treat a false teacher as a brother in Christ and offer fellowship to him, much less give him, quite literally, aid and comfort. Of course many would say this is harsh and unloving (the false brother being the loudest to decry such behavior, no doubt), but our loyalty is to Christ alone. When He limits our hospitality to those who are of the household of God and not those who are teaching falsely about Him and the Scriptures, we are to obey Him and not feign a "better" compassion than He has. To give a warm approval of false teachers is to be guilty of their wickedness and is to help in their deception in the eyes of other Christians, thus misleading them as well. Purity in doctrine and practice is commanded by God.

Mat 7:15  "Beware of the false prophets, who come to you in sheep's clothing, but inwardly are ravenous wolves. 

Mat 7:20  "So then, you will know them by their fruits. 

Mat 7:21  "Not everyone who says to Me, 'Lord, Lord,' will enter the kingdom of heaven, but he who does the will of My Father who is in heaven will enter. 

Mat 7:24  "Therefore everyone who hears these words of Mine and acts on them, may be compared to a wise man who built his house on the rock. 

2Co 6:14  Do not be bound together with unbelievers; for what partnership have righteousness and lawlessness, or what fellowship has light with darkness? 

2Co 6:15  Or what harmony has Christ with Belial, or what has a believer in common with an unbeliever? 

2Co 6:16  Or what agreement has the temple of God with idols? For we are the temple of the living God; just as God said, "I WILL DWELL IN THEM AND WALK AMONG THEM; AND I WILL BE THEIR GOD, AND THEY SHALL BE MY PEOPLE. 

2Co 6:17  "Therefore, COME OUT FROM THEIR MIDST AND BE SEPARATE," says the Lord. "AND DO NOT TOUCH WHAT IS UNCLEAN; And I will welcome you.