13 And whereas you think he is blessed who is undefiled in the way, and walks in His law, you must interpret the former clause by the latter. From the many proofs I have adduced you have learned that no one has been able to fulfil the law. And if the Apostle, in comparison with the grace of Christ, reckoned those things as filth which formerly, under the law, he counted gain, so that he might win Christ, how much more certain ought we to be that the reason why the grace of Christ and of the Gospel has been added is that, under the law, no one could be justified?
Now if, under the law, no one is justified, how is he perfectly undefiled in the way who is still walking and hastening to reach the goal? Surely, he who is in the course, and who is advancing on the road, is inferior to him who has reached his journey's end. If, then, he is undefiled and perfect who is still walking in the way and advancing in the law, what more shall he have who has arrived at the end of life and of the law? Hence the Apostle, speaking of our Lord, says that, at the end of the world, when all virtues shall receive their consummation, He will present His holy Church to Himself without spot or wrinkle, and yet you think that Church perfect, while yet in the flesh, which is subject to death and decay.
You deserve to be told, with the Corinthians, “You are already perfect, you are already made rich: ye reign without us, and I would that you did reign, that we might also reign with you”— since true and stainless perfection belongs to the inhabitants of heaven, and is reserved for that day when the bridegroom shall say to the bride, “You are all fair, my love; and there is no spot in you.” And in this sense we must understand the words: “That ye may be blameless and harmless, as children of God, without blemish”; for He did not say you are, but may be. He is contemplating the future, not stating a case pertaining to the present; so that here is toil and effort, in that other world the rewards of labour and of virtue.
Lastly, John writes: “Beloved, we are sons of God, and it is not yet made manifest what we shall be. We know that when He shall be manifested, we shall be like Him: for we shall see Him even as He is.” Although, then, we are sons of God, yet likeness to God, and the true contemplation of God, is promised us then, when He shall appear in His majesty.
Source: Against the Pelagians (New Advent)