2 John

2 John 11

for the one who gives him a greeting participates in his evil deeds.

This seems like a very harsh pronouncement, that we are not to even offer basic courtesy to someone.  Normally, believers are to be marked by their hospitality, and we are to love everyone who we are near.  Yet here, even something as simple as a greeting is seen as collaboration with Satan.

I guess the key to remember is that this is not merely a greeting to a random person, but to a subversive.  This is an enemy spy, trying to deceive believers into following false doctrine.  As such, any action could be seen by others as an endorsement, and used to trick people into thinking that they should listen and believe what this liar speaks.