Theophylact of Ohrid

This miracle also hints at the universal salvation that was for the entire human race. The ten lepers signify all of human nature, leprous with evil, bearing upon itself the disfigurement of sin, living outside the heavenly city because of its uncleanness, and standing far from God. This very standing far from God interceded for mercy. For the God who loves mankind and desires to save and bless all, the strongest motivation for mercy is to see that no one partakes in goodness. …

Commentary on Luke ↗

Bede

But Jesus answered and said: Were there not ten cleansed? But where are the nine? If one is added to nine, a certain image of unity is completed. It results in such an embrace that the number does not advance further unless it returns again to one, so that this rule through the infinity of numbers is preserved. Therefore, the nine need one, so that they may be unified into a certain form of unity and become ten. But one does not need them to preserve unity. …

On the Gospel of Luke ↗

Bonaventure

Second, as to the rebuke of ingratitude, he adds: But Jesus answering said: Were not ten made clean, and where are the nine? He does not ask about the nine as though ignorant of the person of any one of them, but reproving the irreverence of ingratitude, just as the Lord asked in Genesis 3: "Adam, where are you?" Hence the Gloss: "He inquires where the ungrateful are, as though they were unknown"; so that in them is verified that saying of First Corinthians 14: "He who is ignorant shall be ignored."

Commentary on Luke, Chapter 17 ↗