16 Wherefore did he choose?...“Because God loves mercy and truth”. The Lord loves mercy, by which He first came to my help: He loves truth, so as to give to him that believes what He has promised. Hear in the case of the Apostle Paul, His mercy and truth, Paul who was first Saul the persecutor. He needed mercy, and he has said that it was shown towards him: “I who was before a blasphemer, and a persecutor, and injurious: but I obtained mercy, that in me Christ Jesus might show forth all longsuffering towards those who shall believe in Him unto life eternal.” So that, when Paul received pardon of such great crimes, no one should despair of any sins whatever being forgiven him.
Lo! You have Mercy....Lo, we see that Paul holds Him a debtor, having received mercy, demanding truth. The Lord, he says, shall give back in that day. What shall He give you back, but that which He owes you? How owes He unto you? What have you given Him? “Who has first given unto Him, and it shall be restored to him again.” The Lord Himself has made Himself a debtor, not by receiving, but by promising: it is not said unto Him, Restore what You have received: but, Restore what You have promised.
He has shown mercy unto me, he says, that He might make me innocent: for before I was a blasphemer and injurious: but by His grace I have been made innocent. But He who first showed mercy, can He deny His debt? “He loves mercy and truth. He will give grace and glory.” What grace, but that of which the same one said: “By the grace of God I am what I am”? What glory, but that of which he said, “There is laid up for me a crown of glory”?
Source: The Enarrations, or Expositions, on the Psalms (New Advent)