This is a metonym for eating, which was the usual way people in that day ate meals. Alternate translation: "they were eating a meal" (See: [[rc://en/ta/man/translate/figs-metonymy]])
Jesus is rebuking his disciples because they would not believe in him. Translate this idiom so it is understood that the disciples were not believing Jesus. Alternate translation: "refusal to believe" (See: [[rc://en/ta/man/translate/figs-idiom]])