Jesus calls to conversion. This call is an essential part of the proclamation of the kingdom:"The time is fulfilled, and the kingdom of God is at hand; repent, and believe in the gospel" (Mk 1:15). In the Church's preaching this call is addressed first to those who do not yet know Christ and his Gospel. Also, Baptism is the principal place for the first and fundamental conversion... It is by faith in the Gospel and by baptism that one renounces evil and gains salvation, that is, the forgiveness of all sins and the gift of new life.
Christ's call to conversion continues to resound in the lives of Christians. This second conversion is an uninterrupted task for the whole Church who, "clasping sinners to her bosom, (is) at once holy and always in need of purification, (and) follows constantly the path of penance and renewal" (Vatican II LG 8). This endeavor of conversion is not just a human work. It is the movement of a "contrite heart," (Ps 51,19) drawn and moved by grace to respond to the merciful love of God who loved us first (1Jn 4:10)...
The human heart is heavy and hardened. God must give man a new heart (Ez 36:26f.). 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 of offending God by sin and being separated from him. The human heart is converted by “looking upon him whom our sins have pierced” (cf. Zac 12:10; Jn 19:37).
Catechism of the Catholic Church
§ 1427-1428, 1432