Catechism of the Catholic Church: Article 9:The Ninth Commandment
Part 3. Life in Christ
Section 2
Chapter 2. "You Shall Love Your Neighbor as Yourself"
Article 9
You shall not covet your neighbor's house; you shall not covet your neighbor's wife, or his manservant, or his maidservant, or his ox, or his ass, or anything that is your neighbor's.
Every one who looks at a woman lustfully has already committed adultery with her in his heart.
2514St. John distinguishes three kinds of covetousness or concupiscence: lust of the flesh, lust of the eyes, and pride of life. In the Catholic catechetical tradition, the ninth commandment forbids carnal concupiscence; the tenth forbids coveting another's goods.
2515Etymologically, "concupiscence" can refer to any intense form of human desire. Christian theology has given it a particular meaning: the movement of the sensitive appetite contrary to the operation of the human reason. The apostle St. Paul identifies it with the rebellion of the "flesh" against the "spirit." Concupiscence stems from the disobedience of the first sin. It unsettles man's moral faculties and, without being in itself an offense, inclines man to commit sins.
2516Because man is a composite being, spirit and body, there already exists a certain tension in him; a certain struggle of tendencies between "spirit" and "flesh" develops. But in fact this struggle belongs to the heritage of sin. It is a consequence of sin and at the same time a confirmation of it. It is part of the daily experience of the spiritual battle:
For the Apostle it is not a matter of despising and condemning the body which with the spiritual soul constitutes man's nature and personal subjectivity. Rather, he is concerned with the morally good or bad works, or better, the permanent dispositions - virtues and vices - which are the fruit of submission (in the first case) or of resistance (in the second case) to the saving action of the Holy Spirit. For this reason the Apostle writes: "If we live by the Spirit, let us also walk by the Spirit."
I. Purification of the Heart
2517The heart is the seat of moral personality: "Out of the heart come evil thoughts, murder, adultery, fornication. . . . " 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.
2518The 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. 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."
2519The "pure in heart" are promised that they will see God face to face and be like him. 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.