Catechism of the Catholic Church
2517 The heart is the seat of moral personality: "Out of the heart come evil thoughts, murder, adultery, fornication...." 305 The struggle against carnal covetousness entails purifying the heart and practicing temperance:
Remain simple and innocent, and you will be like little children who do not know the evil that destroys man's life. 306
2518 The sixth beatitude proclaims, "Blessed are the pure in heart, for they shall see God." 307 "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; 308 chastity or sexual rectitude; 309 love of truth and orthodoxy of faith. 310 There is a connection between purity of heart, of body, and of faith:
The faithful must believe the articles of the Creed "so that by believing they may obey God, by obeying may live well, by living well may purify their hearts, and with pure hearts may understand what they believe." 311
2519 The "pure in heart" are promised that they will see God face to face and be like him. 312 Purity of heart is the precondition of the vision of God. Even now it enables us to see according to God, to accept others as "neighbors"; it lets us perceive the human body - ours and our neighbor's - as a temple of the Holy Spirit, a manifestation of divine beauty.
English Translation of the Cathechism of the Catholic Church for the United States of America © 1997, United States Catholic Conference, Inc.