13 But sometimes you show mercy to the poor. I know it as well as you. But even in this again great is the mischief. For you do this either in pride or in vainglory, so as not to profit even by your good deeds. What can be more wretched than this, to be making your shipwreck in the very harbor? To prevent this, when you have done any good action, seek not thanks from me, that you may have God your debtor. For, “Lend,” says He, “unto them from whom you do not expect to receive.”
You have your Debtor; why leave Him, and require it of me, a poor and wretched mortal? What? Is that Debtor displeased, when the debt is required of Him? What? Is He poor? Is He unwilling to pay? Do you see not His unspeakable treasures? Do you see not His indescribable munificence? Lay hold then on Him, and make your demand; for He is pleased when one thus demands the debt of Him. Because, if He see another required to pay for what He Himself owes, He will feel as though He were insulted, and repay you no more; nay, He justly finds fault, saying, “Why, of what ingratitude have you convicted me? What poverty do you know to be in me, that you hasten by me, and resortest unto others? Have you lent to One, and do you demand the debt of another?”
For although man received it, it was God that commanded you to bestow; and His will is to be Himself, and in the original sense, debtor, and surety, affording you ten thousand occasion to demand the debt of Him from every quarter. Do not then let go so great facility and abundance, and seek to receive of me who have nothing. Why, to what end do you display to me your mercy shown to the poor. What! Was it I that said to you, Give? Was it from me that you heard this; that you should demand it back of me? He Himself has said, “He that has pity upon the poor lends to God.” You have lent to God: put it to His account.
“But He does not repay the whole now.” Well, this too He does for your good. For such a debtor is He: not as many, who are anxious simply to repay that which is lent; whereas He manages and does all things, with a view of investing likewise in security that which has been given unto Him. Therefore some, you see, He repays here: some He assigns in the other place.
Source: Homilies on the Gospel of St. Matthew (New Advent)