Youcat commented through CCC – Question n. 312.

YOUCAT Question n. 312 - How does a person know that he has sinned?

(Youcat answer)  A person knows that he has sinned through his conscience, which accuses him and motivates him to confess his offenses to God.

 A deepening through CCC

(CCC 1797) For the man who has committed evil, the verdict of his conscience remains a pledge of conversion and of hope.        

 Reflecting and meditating 

(CCC Comment)

(CCC 1848) As St. Paul affirms, "Where sin increased, grace abounded all the more" (Rom 5:20). 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 our Lord" (Rom 5:21). 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 (John Paul II, Dominum et Vivificantem, 31 § 2).

(The next question is: Why must a sinner turn to God and ask him for forgiveness?)