As with the other examples that Jesus has given in his sermon, he is making use of rhetorical overstatement to emphasize a particular point. In this case, he is not encouraging God’s people to ignore or invite insults. Neither is he prohibiting individuals from acting in self-defence when their lives are threatened by criminals. His focus is on personal relationships between people, affirming that Christians must seek to return good for evil and not come back with bigger and better insults when they are insulted.
39 But I say to you, Do not resist the one who is evil. But if anyone slaps you on the right cheek, turn to him the other also.