1 When our Lord and Saviour Jesus Christ was speaking at length of the coming of the Holy Ghost, He said among the rest, “He shall convince the world of sin, and of righteousness, and of judgment.” Nor when He had said this, did He pass on to another subject; but vouchsafed to convey a somewhat more explicit notice of this same truth. “Of sin,” said He, “because they believed not on Me. Of righteousness, because I go to the Father. Of judgment, because the prince of this world has been judged already.” There arises therefore within us a desire of understanding, why as if it were men's only sin, not to believe in Christ, He said it of this alone, that the Holy Ghost should convince the world; but if it is plain that besides this unbelief there are manifold other sins of men, why of this alone should the Holy Ghost convince the world?
Is it because all sins are by unbelief retained, by faith remitted; that therefore God imputes this one above all the rest, by which it comes to pass that the rest are not loosed, so long as proud man believes not in an Humbled God? For so it is written; “God resists the proud, but gives grace unto the humble.” Now this grace of God is a gift of God. But the greatest gift is the Holy Ghost Himself; and therefore is it called grace. For forasmuch “as all had sinned, and needed the glory of God; because by one man sin entered into the world, and death by his sin in whom all have sinned;” therefore is it grace because given gratuitously.
And therefore is it given gratuitously, because it is not rendered as a reward after a strict scrutiny of deserts, but given as a gift after the pardon of sins.
Source: Sermons on Selected Lessons of the New Testament (New Advent)