Penance has been a theme for me of late, and today I read this section from The Catechism of the Catholic Church, paragraphs 1430-1433.
Jesus' call to conversion and penance, like that of the prophets before him, does not aim first at outward works, "sackcloth and ashes," fasting and mortification, but at the conversion of the heart, interior conversion. Without this, such penances remain sterile and false; however, interior conversion urges expression in visible signs, gestures and works of penance. (Cf. Joel 2:12-13; Is. 1:16-17; Mt. 6:1-6, 16-18)
Interior repentance is a radical reorientation of our whole life, a return, a conversion to God with all our heart, an end of sin, a turning away from evil, with repugnance toward the evil actions we have committed. At the same time it entails the desire and resolution to change one's life, with hope in God's mercy and trust in the help of his grace. This conversion of heart is accompanied by a salutary pain and sadness which the Fathers called animi cruciatus (affliction of spirit) and compunctio cordis (repentance of heart).
The human heart is heavy and hardened. God must give a man a new heart (Cf. Ez. 36:26-27). Conversion is first of all a work of the grace of God who makes our hearts return to him: "Restore us to thyself, O Lord, that we may be restored!" (Lam. 5:21) God gives us the strength to begin anew. It is in discovering the greatness of God's love that our heart is shaken by the horror and weight of sin and begins to fear offending God by sin and being separated from him. The human heart is converted by looking upon him whom our sins have pierced (Cf. Jn. 19:37, Zech 12:10).
"Let us fix our eyes on Christ's blood and understand how precious it is to his Father, for, poured out for our salvation, it has brought to the whole world the grace of repentance." (St. Clement of Rom, Ad Cor. 7.4: PG 1, 224)
Since Easter, the Holy Spirit has proved "the world wrong about sin," (Cf. Jn. 16:8-9) i.e. proved that the world has not believed in him whom the Father has sent But this same Spirit who brings sin to light is also the Consoler who gives the human heart grace for repentance and conversion (Cf. Jn. 15:26; Acts 2:36-38; John Paul II, DeV 27-48).