Matthew 5:8

The clean of heart are either those who give themselves to the practice of every virtue, and are conscious to themselves of no evil, or those who are adorned with the virtue of charity. For nothing is so necessary as this purity in such as desire to see God. Keep peace with all and chastity, says St. Paul, for without this none can see God. Many are merciful to the poor and just in their dealings, but abstain not from luxury and lust. Therefore our Saviour, wishing to shew that mercy was not sufficient, adds, that if we would see God, we must also be possessed of the virtue of purity. (St. John Chrysostom, hom. xv.) By this, we shall have our heart exempt from all disordinate love of creatures, and shall be exclusively attached to God. (Haydock) --- The clean of heart, i.e. they who are clean from sin: who are pure in body and mind, says St. John Chrysostom. It seems to be a particular admonition to the Jews, who were mostly solicitous about an outward and legal cleanness. (Witham)