Please Read 2 John 1:9-11
God says anyone who does not continue in the doctrine of Christ does not have God. Whoever does follow his teaching has both the Father and the son. People who come to you with a different doctrine are to be shunned. We shouldn't welcome them at all. God says don't let them into your house or even say a greeting to them. Anyone who does is a sharer in their evil deeds. Let's examine this for a moment.
To some this seems very harsh. But not when it comes to what's important to them. To illustrate; let's say you're a parent. For many years you teach your children a set of values, things you know will help them make it through life. Then some unscrupulous person(s) comes along and bewitches your children with fancy words and promises. They turn your children completely against everything you taught them. They even turn them against you their parent. How would that make you feel? Now you know God feels at 2 John 1:9-11.
When someone turns folks away from Christ by teaching false doctrines they are helping them go to hell. This is serious. Jesus is the truth, the way, and the life. No one comes to the Father except through him. (John 14:6) Hence our loving Father warns us to completely shun those who would dissuade us from him, and not to listen to any of their rhetoric. Sometimes this gets tricky because many proclaiming to follow Jesus are the very ones spreading untruth. They are being used by the Devil to enslave and kill people. When you really think about it, is avoiding these false teachers too harsh?
DON'T GREET THEM
Again, if you knew of individuals that messed up your children's minds so bad that they threw away everything you taught them, and they now hate even you, would you have a nice warm greeting for them? Of course not. So God is not unreasonable when he says not to even greet those who's business is to get us to believe anything other than the true doctrine of Christ. These people are instrumental in damning many souls forever. There are no warm welcoming greetings or feelings for such people by God or his servants. Anyone who welcomes these soul assassins are sharers in their evil work God says.