“But the things which proceed out of the mouth come forth out of the heart; and they defile the man.”
On the other hand the things which can really defile a man religiously and morally (parallel ideas in those days) are the things that are revealed by what comes from the mouth in the words that a man speaks (compare Matthew 12:36-37). For it is they which come from the heart, and demonstrate what is in the heart. They, as it were, reproduce what is in the inner man.
‘Out of the mouth.' This is in contrast with what goes in at the mouth earlier. What comes out of the mouth reveals the evil thoughts of men. The list that follows does not specifically keep the mouth in mind, but Jesus was well aware that in the end all these sins would in one way or another result in words which would reveal an evil heart.