15 Then answered Peter and said unto him, Declare unto us this parable. 16 And Jesus said, Are ye also yet without understanding? 17 Do not ye yet understand, that whatsoever entereth in at the mouth goeth into the belly, and is cast out into the draught? 18 But those things which proceed out of the mouth come forth from the heart; and they defile the man. 19 For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies: 20 These are the things which defile a man: but to eat with unwashen hands defileth not a man. (Matthew 15:15-20 KJV)
William Burkitt’s Commentary
The disciples desiring the interpretation of the foregoing parable, our Saviour gives it them; but with all expostulates with them, that they did not understand a thing so obvious and plain: Are ye yet without understanding? As if he had said, “Have ye sat thus long under my ministerial teaching, and enjoyed the benefit of my company and conversation, and are yet no farther proficients in knowledge?”
Whence learn, that our Lord expects a proficiency in knowledge from us, answerable to the opportunities and means of knowledge enjoyed by us.
Next, he gives them the sense and signification of the parable; telling them, that it is out of a sinful heart that all sin proceeds; the heart is the cage or nest, which is full of these unclean birds, and from whence they take their flight. Though the occasions of sin are from without, yet the source and origin of sin is from within.
Learn, that the heart of man is the sink and seed-plat of all sin, and the fountain of all pollution; the life could not be so bad, if the heart were not worse; all the irregularity of our lives flows from the impurity of our hearts and natures.