Thanks to Brennus for the full translation below, taken from St. Augustine's Questions on the Gospel, 2, 40.
Italicized parts are Brennus' own comments.
In the 10 lepers whom the Lord cleansed whenhe said "Go show yourselves to the priests" there is mucn to be asked which rightly motivates those who inquire. Not only concerning the number -- what it might mean that there are 10 and that one of them alone gave thanks, these (questions) can indeed freely be asked, not indeed that (these things) are investigated, but that nothing or little impede the intention of the readers -- But moreso than that, why he sent them to the priests so that when they went they would be cured.
No one of those to whom he performed corporal benefits is he found to have sent to the priests unless they were lepers. And indeed,he cleansed him of leprosy to whom he said "Go, show thyself to the priests and offer for theyself the sacrifice which Moses prescribed in his testimony" (Luc V 13,14.) Therefore what sort of spiritual cleansing can be understood of them whom he had refuted to have stood out ungrateful?
Concerning the body, indeed it is easy to be able to see a man to not have leprosy and, however, to not be of a good mind. Regarding the signification of this miracle however, it stirs up those considering (it), how can the ungrateful be said to be clean?
It is to be asked therefore what leprosy itself will signify. Indeed , they are not said to be healed but cleaned, those who have suffered it. Indeed, the defect is of the color, not the strength and integrity of the senses and members. Therefore, it is not absurd to understand the lepers to be (he means symbolically here) those who, not having knowledge of the true faith, profess various doctrines of error. Indeed, or else they are hiding their inexperience but professing the sum of their experience in the light, they show themselves (to be) boasting by their conversation. Further, there is no false doctrine that does not mix in some truth. The true therefore is inordinately mixed with the false in one disputation or narration of a man, just as in the appearance by the color of one body, leprosy is indicated, just as it changes and stains human bodies with true and false hues of color. (Color was a lot more important to ancient people than to us in the artificiality of moder times. Augustine would have thought of a body as having a "true" color and leprosy was not it.)
These however are so much to be shunned in the Church (false teachers, not lepers necessarily) that if it is possible, further removed, they interrupt Christ with a commotion, just as the ten stood at a distance and lifted up their voices crying "Jesus, teacher, have mercy on us," Indeed, and in that they called him by "teacher," by which name I know not whether anyone interrupted the Lord for corporal medicine, it is satisfactory, I think, to indicate the leprosy of false doctrine, which the good teacher wipes away.
Truly, hardly anyone of the faith doubts the priesthood of the Jews to be a figure of the royal priesthood to come which is in the church, by which all who are connected to the body of Christ are consecrated, the high and true priest of priests. For now all are annointed while formerly it was done to Kings and priests, and which Peter said to the Christian people, writing "royal priesthood" (I Pet. II, 19) and whether he declared that name to be fitting to that people to which that annointing pertained.
Moreover therefore, the Lord cured and corrected defects of health as if of the soul of the members as well as the senses, just as more inwardly by the conscience and understanding.
Truly, doctrine, whether of imbuing through the sacraments, or of catechizing through spoken sermons, as well as reading, where a certain color is understood true and sincere, because it is easily evident outside (not indeed in hidden thoughts but by manifest works these (I think he means the sacraments here) are done) is appropriately joined with the Church. Therefore Paul heard by the audible voice of the Lord "why does thou persecute me?" and "I am Jesus whom thou persecutest." However he sent him to Annias that by the priesthood which is established in the Church he would take possession of the sacrament of the true faith and that his true color would be proven. Not because the Lord can not do all things himself - for who else is it that does everything in the Church - but that the society itself of the congregation of faith, by approving each other as well as by communicating the doctrine of the true faith, in all which is said by words or signified by the sacraments, just as it draws forth one aspect of the true color. To this pertains what the apostle himself said "Then after 14 years I again went up to Jerusalem with Barnabas taking Titus with me. And I went according to revelation, and communicated to them the gospel that I preach among the gentiles, but apart to them who seemed to be something, lest I perhaps should run or had run in vain . . . and when they had known the grace that had benn given to me, James and Kepha and John, who were reputed to be pillars, gave to me and Barnabas the right hand of fellowship that we shouold go unto the gentiles and they unto the circumcision." (Gal II 1,2,9).
This collection (of passages) itself demonstrates one aspect of doctrine exempt from all variation which indeed he safely warns the Corinthians saying "Now I beseech you brethren by the name of our Lord Jesus Christ that you all speak the same thing and that there be no schisms among you, but that you be perfect in the same mind and in the same judgement." (I Cor I 10) and Cornelius when "while Peter was yet speaking these words the Holy Ghost fell on all them that heard the word" (Acts X 44) which in keeping (with what I have been saying) now it is easy to see that, it is possible that everyone in the society of the Church followed the intact and true doctrine, and explained everything following the rule of Catholic faith, that he distinguish the creature from the Creator and by him (the Creature) is manifested by the variation of lies, just as he had suffered leprosy. (this next sentence doesn't really make sense to me. I'm sorry) And however, the ungrateful one would be (so) to God and to the Lord, the one who had made him clean. Because he was elated by pride he is not stirred by the pius humility of graces to be given and he is compared to them of whom the apostle said "Who when they have known God they did not glorify God or give thanks" (Rom I 21). Because indeed he said they knew God, he showed them indeed to have been cleansed of leprosy, but indeed immediately accused them of being ungrateful.
And on that account, such remain just as the imperfect ones, in the enumeration of the nine. If indeed one is added to the nine, a certain likeness of unity is filled from which it should be so complete that no further number can proceed unless it returns to one, and this rule is preserved though the infinity of numbers. That nine indeed lacks one, by a certain form of unity, it is coagulated that they be ten. One, however, needs nothing of them that is preserve its unity. On which account that the nine who did not give thanks are made reprobate, they are closed from the cnsortium of unity, thus the one who gave thanks, by signification of the Church is approved and praised. And because they were Jews, they are declared to have been lost through pride to the Kingdom of Heaven, where the greatest unity is preserved. The one indeed who was a Samaratine - which is translated "Guard" (I don't know if this etymolgy is correct, maybe it is - Brennus) - to him from whom he received he paid what he received and ina certain measure singing that in the Psalm "My strength I preserve to thee" (Psalm LVIII 10) through the action of grace he is subject to the king and by humble devotion preserves the unity of the kingdom.