When the Pharisees with some scribes who had come from Jerusalem gathered around Jesus, they observed that some of his disciples ate their meals with unclean, that is, unwashed, hands. (For the Pharisees and, in fact, all Jews, do not eat without carefully washing their hands, keeping the tradition of the elders. And on coming from the marketplace they do not eat without purifying themselves. And there are many other things that they have traditionally observed, the purification of cups and jugs and kettles and beds.) So the Pharisees and scribes questioned him, “Why do your disciples not follow the tradition of the elders but instead eat a meal with unclean hands?” He responded, “Well did Isaiah prophesy about you hypocrites, as it is written:
This people honors me with their lips, but their hearts are far from me; In vain do they worship me, teaching as doctrines human precepts.
You disregard God's commandment but cling to human tradition.” He went on to say, “How well you have set aside the commandment of God in order to uphold your tradition! For Moses said, Honor your father and your mother, and Whoever curses father or mother shall die. Yet you say, ‘If someone says to father or mother, “Any support you might have had from me is qorban$2"0$ (meaning, dedicated to God), you allow him to do nothing more for his father or mother. You nullify the word of God in favor of your tradition that you have handed on. And you do many such things.”
The sense of the word "honour" in Scripture is not so much the saluting and paying court to men, but alms-giving, and bestowing gifts; as St. Paul says, "Honour widows who are widows indeed." (1 Tim 5:3)
Notwithstanding the existence of such a divine law, and the threats against such as break it, you lightly transgress the commandments of God, observing the traditions of the Elders. For the Pharisees, wishing to devour the offerings, instructed sons, when their parents asked for some of their property, to answer them, what you have asked of us is qorban, that is, a gift already dedicated to God; thus the parents would not require it, as being offered up to the Lord, and in that way profitable for their own salvation. Thus the Pharisees deceived the sons into neglecting their parents, while they themselves devoured the offerings; with this therefore the Lord reproaches them, as transgressing the law of God for the sake of personal gain.
Wherefore it goes on, "You nullify the word of God in favor of your tradition that you have handed on. And you do many such things;" transgressing, that is, the commands of God, that you may observe the traditions of men.
Catena Aurea (St. Thomas Aquinas)
On the Gospel of Mark (St. Bede the Venerable)
Explanations on the New Testament (St. Theophylact of Ochrid)