John Gill Commentary Matthew 15:18

"But the things which proceed out of the mouth come forth out of the heart; and they defile the man." — Matthew 15:18 (ASV)

But those things which proceed out of the mouth
Meaning not material things, as spittle, vomit but, as it follows, which

come forth from the heart :
are first conceived and formed there, and then come forth from there, and are expressed by the mouth; as all idle words, foolish talking, filthy jesting, unsavoury communication, and every word that is rotten and corrupt, or which is done in the life and conversation;

and they defile the man :
the heart is the corrupt fountain from which all moral defilement flows; and sinful words and actions are the impure streams, which spring from there, and increase the moral pollution of human nature.