I have long been bothered by the unnecessarily robust verb "declare" here, and the unduly forthright "Thus he declared all foods clean." This very common translation gives the impression that Mark is flagging up his comments here far more blatantly than he is. The translation is based on a reading of Mark 7.18-19 that construes καθαρίζων πάντα τὰ βρώματα at the end of Jesus' speech of 7.19 with καὶ λέγει αὐτοῖς at the beginning of 7.18, thus "Jesus says to them . . . cleansing all foods," or "Jesus says, cleansing all foods, 'Are you too without understanding . . .'" The reason that "Thus he declared . . . " comes in is that translators are anxious about bringing forward the end of one verse (19) to the beginning of another (18) -- it breaks the verse-by-verse rule of Bible translation, without which people would not be able to look up the verses they wanted in the right place. And given the necessity to keep words in sequence, the decision is made to translate this participle phrase by adapting the force of λέγει and turning the whole into a fresh, interpretative clause. But that move, from a clarificatory parenthesis to a stand-alone statement, and the use of the word "declare", alters the force of the original.
Now it may be that the standard translation picks up the sense of this interpretative clause, even if it overstates it, but even there I am not sure. Could the person orally delivering Mark 7.18-19 have made this intelligible?:
He says to them, "Then do you also fail to understand? Do you not see that whatever goes into a person from outside cannot defile, since it enters, not the heart but the stomach, and goes out into the sewer?", cleansing all foods.I am not convinced that we are reading Mark right here.