As St. Paul affirms, "Where sin increased, grace abounded all the more." But to do its work grace must uncover sin so as to convert our hearts and bestow on us "righteousness to eternal life through Jesus Christ ourLord." Like a physician who probes the wound before treating it, God, by his Word and by his Spirit, casts a living light on sin:
Conversion requires convincing of sin; it includes the interior judgment of conscience, and this, being a proof of the action of the Spirit of truth in man's inmost being, becomes at the same time the start of a new grant of grace and love: "Receive the Holy Spirit." Thus in this "convincing concerning sin" we discover a double gift: the gift of the truth of conscience and the gift of the certainty of redemption. The Spirit of truth is the Consoler.
Source: Catechism of the Catholic Church (Vatican.va)