“Do not ye yet understand, that whatsoever entereth in at the mouth goeth into the belly, and is cast out into the draught?”
Matthew 15:17 — Greek Interlinear
Church Fathers on Matthew 15:17
The Lord was used to speak in parables, so that Peter when he heard, 'That which entereth into the mouth, defileth not a man,' thought it was spoken as a parable, and asked, as it follows; 'Then answered Peter, and said unto him, Declare unto us this parable.' And because he asked this on behalf of the rest, they are all included in the rebuke, 'But he said, Are ye also yet without understanding?'
He is reproved by the Lord, because He supposed that to be spoken parabolically, which was indeed spoken plainly. Which teaches us that the hearer is to be blamed who would take dark sayings as clear, or clear sayings as obscure.
Or, The Lord blames him, because it was not from any uncertainty that he asked this, but from offence which he had taken. The multitudes had not understood what had been said; but the disciples were offended at it, whence at the first they had desired to ask Him concerning the Pharisees, but had been stayed by that mighty declaration, 'Every plant, &c.'