A.T. Robertson Commentary 2 John 1:11

"for he that giveth him greeting partaketh in his evil works." — 2 John 1:11 (ASV)

Partaketh in his evil works (κοινωνε τοις εργοις αυτου τοις πονηροις). Associative instrumental case with κοινωνε as in 1 Timothy 5:22, common verb from κοινωνος (partner). It is to be borne in mind that the churches often met in private homes (Romans 16:5; Colossians 4:15), and if these travelling deceivers were allowed to spread their doctrines in these homes and then sent on with endorsement as Apollos was from Ephesus to Corinth (Acts 18:27), there was no way of escaping responsibility for the harm wrought by these propagandists of evil. It is not a case of mere hospitality to strangers.