Theophylact of Ohrid

Christ sends him to the priest because, by the commandment of the Law, a leper could not enter the city unless the priest declared him cleansed of his leprosy; otherwise he had to be expelled from the city. At the same time, the Lord commands him also to bring a gift, as those who were cleansed customarily offered: this as a testimony that He is not an opponent of the Law but, on the contrary, values it so highly that what the Law commands, He too orders to be fulfilled.

Commentary on Mark ↗

Bede

But go and show yourself to the chief priests, etc. He is ordered to show himself to the priest so that the priest might understand that he was cured not by the order of the law, but by the grace of God above the law. However, he is to offer a sacrifice to show that the Lord did not overthrow the law but fulfilled it, who, walking according to the law, healed those above the law whom the remedies of the law had not healed. And he well added: As a testimony to them, that is, if they believe in God, if the leprosy of impiety departs. …

On the Gospel of Mark ↗