He said unto them, "Go and show yourselves unto the priests."

And why did He not rather say, "I will, be you cleansed;" as he did in the case of another leper: but commanded them rather to show themselves unto the priests? It was because the law gave directions to this effect to those who were delivered from leprosy: for it commanded them to show themselves to the priests, and to offer a sacrifice for their cleansing. He commanded them therefore to go, as being already healed, and, that they might, so to speak, bear witness to the priests, as the rulers of the Jews, and ever envious of His glory, that wonderfully, and beyond their hope, they had been delivered from their misfortune by Christ's willing that they should be healed. He did not heal them first, but sent them to the priests, because the priests knew the marks of leprosy, and of its being healed. He sent them to the priests, and with them He sent also the healing. What however was the law of leprosy, and what the rules for its purification, and what the meaning of each of the particulars commanded by the law, we have more fully described at the commencement of our Savior's miracles as recorded by Luke, and referring thither such as are anxious for learning, let us now proceed to what follows. The nine then, as being Jews, falling into a thankless forgetfulness, did not return to give glory to God: by which He shows that Israel was hard of heart, and utterly unthankful: but the stranger,---for as being a Samaritan he was of foreign race, having been brought thither from Assyria: for the phrase is not without meaning, ”in the middle of Samaria and Galilee:" ----returned with a loud voice to glorify God. It shows therefore that the Samaritans were grateful, but that the Jews, even when benefited, were ungrateful.

Commentary on the Gospel of Luke
Sermon 116
By St. Cyril of Alexandria