The short answer
Jesus begins His public preaching with a call to repentance. Conversion is not optional decoration around faith; it is part of our response to the Gospel.
Jesus said
Christ calls sinners to turn back, receive mercy, and begin a transformed life.
Luke 13:3Luke 15:7Luke 19:1–10
The apostles taught
At Pentecost, Peter's first answer to those cut to the heart begins with repentance and leads directly to baptism.
Acts 2:37–38Acts 3:192 Corinthians 7:10
The Catholic synthesis
Conversion continues throughout Christian life. It includes sorrow for sin, confession, reparation, and a renewed desire for holiness.