Albert Barnes on Mark 7:17

The parable. The obscure and difficult remarks which he had made in Mk 7:15. The word parable, here, means obscure and difficult saying. They could not understand it. They had probably imbibed many of the popular notions of the Pharisees, and they could not understand why a man was not defiled by external things. It was, moreover, a doctrine of the law, that men were ceremonially polluted by contact with dead bodies, etc., and they could not understand how it could be otherwise.

(v) "And when" Mt 15:15