Catechism of the Catholic Church §2518 — excerpt
The sixth beatitude proclaims, "Blessed are the pure in heart, for they shall see God." "Pure in heart" refers to those who have attuned their intellects and wills to the demands of God's holiness, chiefly in three areas: charity; chastity or sexual rectitude; love of truth and orthodoxy of faith.
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.
Also cites: St. Augustine, Defide · symbolo 10, 25: PL 40, 196