Jesus had just said something the disciples found puzzling, and they asked Him to explain. His first question expresses surprise that even they had missed it. The word "also" sets them beside the others who did not understand. Matthew Henry hears it this way: "Are ye dull also, as dull as the people that cannot understand, as dull as the Pharisees that will not?" Gill says Jesus spoke with some warmth of displeasure. Barnes, on the parallel in Matthew 15:16, says Jesus "appeals, in explaining this, to their common sense," wondering that they had not yet learned to judge the traditions by common sense and by His own instruction. His second question presses them to reason it through: food from outside cannot defile a person.

Jesus answered His own question. Food "entereth not into his heart, but into the belly, and goeth out into the draught, purging all meats" (Mark 7:19). Then He named what truly defiles: "That which cometh out of the man, that defileth the man" (Mark 7:20). "From within, out of the heart of men, proceed evil thoughts," and a long list follows, from adulteries and murders to pride and foolishness (Mark 7:21-22). "All these evil things come from within, and defile the man" (Mark 7:23).

The disciples called Jesus' saying a "parable" (Mark 7:17). Barnes explains that the word here means an "obscure" and "difficult saying." The Greek for "without understanding" is asynetoi, which Thayer defines as "unintelligent, without understanding." The word for "defile" is koinoo, literally to make common; Thayer explains it in this passage as to make (levitically) unclean, the kind of uncleanness the washing rules were meant to prevent.

In the next verse, "the draught" is, in Barnes' words, "The sink, the vault," and "purging" means to purify or cleanse. The ESV renders the end of Mark 7:19 as "(Thus he declared all foods clean.)" Barnes says food "cannot render his soul polluted; cannot make him a sinner, so as to need this purifying."

Jesus' question shows both His honesty and His patience. He did not pretend the disciples understood when they did not, but He also did not leave them in the dark. He explained. Matthew Henry notes that "Some truths prove themselves, if they be but rightly explained and apprehended."

The truth He explained goes to the root of the human problem. Uncleanness is not something that gets on us from outside; it comes out of us. Matthew Henry's Concise Commentary puts it plainly: "Our wicked thoughts and affections, words and actions, defile us, and these only." This is why outward washing could never make anyone clean before God. The heart itself needs cleansing. As Matthew Henry puts it, "Our care must therefore be, to wash our heart from wickedness."