Luke 17:19

And he is commanded to rise and walk, because he who, knowing his own weakness, lies lowly on the ground, is led to advance by the consolation of the divine word to mighty deeds. But if faith made him whole, who hurried himself back to give thanks, therefore does unbelief destroy those who have neglected to give glory to God for mercies received. Wherefore that we ought to increase our faith by humility, as it is declared in the former parable, so in this is it exemplified in the actions themselves.

Wherein it is shown, that strangers were more ready to receive the faith, but Israel was slow to believe; and so it follows, And he said unto him, Arise, go thy way, thy faith has made thee whole.

After speaking the foregoing parable, our Lord censures the ungrateful...

(de Quæst. Ev. l. ii. qu. 40.) The lepers may be taken mystically for those who, having no knowledge of the true faith, profess various erroneous doctrines. For they do not conceal their ignorance, but blazen it forth as the highest wisdom, making a vain show of it with boasting words. But since leprosy is a blemish in colour, when true things appear clumsily mixed up with false in a single discourse or narration, as in the colour of a single body, they represent a leprosy streaking and disfiguring as it were with true and false dyes the colour of the human form. Now these lepers must be so put away from the Church, that being as far removed as possible, they may with loud shouts call upon Christ. But by their calling Him Teacher, I think it is plainly implied that leprosy is truly the false doctrine which the good teacher may wash away. Now we find that of those upon whom our Lord bestowed bodily mercies, not one did He send to the priests, save the lepers, for the Jewish priesthood was a figure of that priesthood which is in the Church. All vices our Lord corrects and heals by His own power working inwardly in the conscience, but the teaching of infusion by means of the Sacrament, or of catechizing by word of mouth, was assigned to the Church. And as they went, they were cleansed; just as the Gentiles to whom Peter came, having not yet received the sacrament of Baptism, whereby we come spiritually to the priests, are declared cleansed by the infusion of the Holy Spirit. Whoever then follows true and sound doctrine in the fellowship of the Church, proclaiming himself to be free from the confusion of lies, as it were a leprosy, yet still ungrateful to his Cleanser does not prostrate himself with pious humility of thanksgiving, is like to those of whom the Apostle says, that when they knew God, they glorified him not as God, nor were thankful. (Rom. 1:21.) Such then will remain in the ninth number as imperfect. For the nine need one, that by a certain form of unity they may be cemented together, in order to become ten. But he who gave thanks was approved of as a type of the one only Church. And since these were Jews, they are declared to have lost through pride the kingdom of heaven, wherein most of all unity is preserved. But the man who was a Samaritan, which is by interpretation "guardian," giving back to Him who gave it that which he had received, according to the Psalm, My strength will I preserve for thee, (Ps. 59:9.) has kept the unity of the kingdom with humble devotion.