Theophylact of Ohrid

But the Samaritan, although he was of a foreign race, returned and expressed his gratitude (for the Samaritans were Assyrians), so that none of the Gentiles would despair, and none of those descended from holy ancestors would boast of this.

Commentary on Luke ↗

Bede

And he was a Samaritan. Indeed, Samaritan is interpreted as custodian. By this name, that people who, by giving thanks to Him from whom they received, attributes everything they received, is most aptly signified, in a manner singing from the Psalm: My strength I will guard for You, because You, O God, are my protector; my God, His mercy will precede me (Psalm LVIII). He falls moreover on his face, because he blushes from the evils he remembers having committed. For there man falls where he is ashamed. …

On the Gospel of Luke ↗

Bonaventure

And he fell on his face before his feet, giving thanks to him, namely through the display of reverence and honor: Ezekiel 2: "I saw and fell upon my face"; and Revelation 1: "When I had seen him, I fell at his feet as though dead." And this one was a Samaritan, to express the figure and signification. For this man, because he was a Samaritan and a foreigner, signifies the gentiles returning to the worship of Christ, just as that Samaritan woman, John 4. …

Commentary on Luke, Chapter 17 ↗