2 John 1:11

1:11 because the person who gives him a greeting shares in his evil deeds. ¹ 

2 John 1:10

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 any greeting, ² 

[1:11]  ¹ sn Shares in his evil deeds. Giving a public greeting could be understood by an onlooker to suggest agreement with the (false) teaching of the opponents and is thus prohibited by John.

[1:10]  ² sn Do not give him any greeting does not mean to insult the person. It means “do not greet the person as a fellow Christian” (which is impossible anyway since the opponents are not genuine believers in the author’s opinion).