And Peter answering said. As the disciples betray excessive ignorance, Christ justly reproves and upbraids them for being still void of understanding, and yet does not fail to act as their teacher. What Matthew ascribes in a peculiar manner to Peter is related by Mark, in the same sense, as a question put by them all; and this is evident from Christ's reply, in which he reproves the ignorance, not of Peter only, but of all of them alike. The general meaning is, that men are not polluted by food, but that they have within themselves the pollution of sins, which afterwards shows itself in the outward actions. Is it objected that intemperance in eating is defilement? The solution is easy. Christ speaks only of the proper and lawful use of those things which God has put in our power. To eat and drink are things in their own nature free and indifferent: if any corruption be added, it proceeds from the man himself, and therefore must be regarded not as external, but internal. [1]

Footnotes

1 - "Et pourtant le vice est tousiours interieur, et ne vient point d'ailleurs;" -- "and therefore sin is always internal, and does not come from without."

See also Mark 7:17-23.
Then answered Peter, and said unto him, Declare unto us this parable - See the notes at Matthew 13:3. The word "parable" sometimes means a dark or obscure saying, Psalm 78:2. Peter meant, "Explain to us more fully this obscure and novel doctrine." To us, now, it is plain; to the disciples, just coming out of Judaism, the doctrine of Jesus was obscure. Mark says that the disciples asked him. There is no contradiction. The question was put by Peter in the name of the disciples; or several of them put the question, though Matthew has mentioned only one. An omission is not a contradiction.

Declare unto us this parable - Is it not strange to hear the disciples asking for the explanation of such a parable as this! The true knowledge of the spirit of the Gospel is a thing more uncommon than we imagine, among the generality of Christians, and even of the learned.

Then answered Peter,.... Mark says, "his disciples asked him concerning the parable"; which might be by the mouth of Peter; who, probably, being the eldest man, and very forward to speak, was generally their spokesman: and who, at this time, might be requested, by the rest, to ask the meaning of the parable, which had given offence to the Pharisees, and was not clearly understood by them; which he accordingly did:
and said unto him, declare unto us this parable; that not what goes into the mouth, but what comes out of it, defiles the man; which, though expressed in very plain words, and easy to be understood, yet did not appear clear to their understandings; and seemed to be contrary, not only to the traditions of the elders, but to the laws of God, respecting the difference of clean and unclean meats; and therefore call it a "parable", and desire an explanation of it.

Then answered Peter and said unto him, Declare unto us this parable--"when He was entered into the house from the people," says Mark (Mark 7:17).

Declare unto us the parable. The figure was used in Matthew 15:11.