Catechism of the Catholic Church §1430 — excerpt
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.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Whole chapters cited: Matthew 16 · Matthew 17 · Matthew 18