John Chrysostom — as excerpted in the Catena Aurea on Matthew 15:15-20

“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. But Peter, who is ever zealous, is not silent even so; therefore the Lord reproves him, adding a reason for His reproof, Do ye not understand, that whatsoever entereth in at the mouth goeth into the belly, and is cast out into the draught?”