John Calvin Commentary Matthew 12:3

"But he said unto them, Have ye not read what David did, when he was hungry, and they that were with him;" — Matthew 12:3 (ASV)

Have you not read what David did? Christ employs five arguments to refute their slander. First, he defends his disciples by citing the example of David, (1 Samuel 21:6). While David was fleeing from Saul’s rage, he sought provisions from the high priest Ahimelech; and as no ordinary food was available, he succeeded in obtaining some of the holy bread. If David’s necessity excused him, the same argument should apply to others. It follows, then, that the ceremonies of the Law are not violated when there is no compromise of godliness.⁷⁷

Now Christ assumes that David was free from blame, because the Holy Spirit commends the priest who allowed him to eat the holy bread. When he says that it was not lawful to eat that bread except for the priests alone, we must understand that he is referring to the ordinary law:

they shall eat those things wherewith the atonement was made,
to consecrate and to sanctify them; but a stranger shall not
eat thereof, because they are holy, (Exodus 29:33).

If David had attempted to do what was against the law, it would have been useless for Christ to cite his example. For no necessity could make lawful what had been prohibited for a specific purpose.

⁷⁷ “Quand on ne derogue rien a la reverence deue, a Dieu;” — “when nothing is taken away from the reverence that is due to God.”;” — “when nothing is taken away from the reverence that is due to God.”