13 Thus, moreover, the Apostle Paul, after shipwrecks, after scourgings, after many and grievous tortures of the flesh and body, says that he is not grieved, but benefited by his adversity, in order that while he is sorely afflicted he might more truly be proved. “There was given to me,” he says, “a thorn in the flesh, the messenger of Satan to buffet me, that I should not be lifted up: for which thing I besought the Lord thrice, that it might depart from me; and He said unto me, My grace is sufficient for you, for strength is made perfect in weakness.” When, therefore, weakness and inefficiency and any destruction seize us, then our strength is made perfect; then our faith, if when tried it shall stand fast, is crowned; as it is written, “The furnace tries the vessels of the potter, and the trial of tribulation just men.” This, in short, is the difference between us and others who know not God, that in misfortune they complain and murmur, while adversity does not call us away from the truth of virtue and faith, but strengthens us by its suffering.
Source: The Treatises of Cyprian (New Advent)