17 Now this election the Apostle demonstrating to be, not of merits going before in good works, but election of grace, says thus: “And in this time a remnant by election of grace is saved. But if by grace, then is it no more of works, otherwise grace is no more grace.” This is election of grace; that is, election in which through the grace of God men are elected: this, I say, is election of grace which goes before all good merits of men. For if it be to any good merits that it is given, then is it no more gratuitously given, but is paid as a debt, and consequently is not truly called grace; where “reward,” as the same Apostle says, “is not imputed as grace, but as debt.” Whereas if, that it may be true grace, that is, gratuitous, it find nothing in man to which it is due of merit, (which thing is well understood in that saying, “You will save them for nothing,”) then assuredly itself gives the merits, not to merits is given.
Consequently it goes before even faith, from which it is that all good works begin. “For the just,” as is written, “shall live by faith.” But, moreover, grace not only assists the just, but also justifies the ungodly. And therefore even when it does aid the just and seems to be rendered to his merits, not even then does it cease to be grace, because that which it aids it did itself bestow. With a view therefore to this grace, which precedes all good merits of man, not only was Christ put to death by the ungodly, but “died for the ungodly.” And ere that He died, He elected the Apostles, not of course then just, but to be justified: to whom He says, “I have chosen you out of the world.”
For to whom He said, “You are not of the world,” and then, lest they should account themselves never to have been of the world, presently added, “But I have chosen you out of the world;” assuredly that they should not be of the world was by His own election of them conferred upon them. Wherefore, if it had been through their own righteousness, not through His grace, that they were elected, they would not have been chosen out of the world, because they would already not be of the world if already they were just.
And again, if the reason why they were elected was, that they were already just, they had already first chosen the Lord. For who can be righteous but by choosing righteousness? “But the end of the law is Christ, for righteousness is to every one that believes. Who is made unto us wisdom of God, and righteousness, and sanctification, and redemption: that, as it is written, He that glories, let him glory in the Lord.” He then is Himself our righteousness.
Source: On Patience (New Advent)