Matthew 21:23

The chief priests and the elders. — St. Matthew and St. Luke add “the scribes,” thus including representatives of the three constituent elements of the Sanhedrin. The character of the teaching is further specified by St. Luke, “as He was preaching the gospel” — proclaiming, i.e., the good news of the kingdom, the forgiveness of sins, and the law of righteousness.

By what authority...? — The right to take the place of an instructor was, as a rule, conferred by the scribes, or their chief representative, on one who had studied “at the feet” of some great teacher, and been solemnly admitted (the delivery of a key, as the symbol of the right to interpret, being the outward token) to that office. The question implied that those who asked it knew that the Prophet of Nazareth had not been so admitted. The second question gave point to the first. Could He name the Rabbi who had trained Him, or authorised Him to teach?