This purity is the indispensable condition for contemplating God in Heaven, to see Him on earth and to know Him by the light of faith.
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. 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.” (CCC 2518)
The “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. (CCC 2519)