Blessed are the pure in heart, for they will see God. Matthew 5:8
“The pure of heart are those who have gotten rid of sin’s filth, have cleansed themselves of all the pollution of the flesh and have pleased God through works for faith and justice. As David testifies in a psalm, “Who will climb up the Lord’s mountain, or who will stand in his holy place? The one with innocent hands and a pure heart, who has not received his soul in vain.” And David, rightly knowing that God can be seen only with a pure heart, prays as follows in the psalm, “Create in me a clean heart, O God, and renew a right spirit within me.” (Psalm 51:10).
Chromatius (c. 400) in Tractate on Matthew 17.6.3-4.