Augustine of Hippo

"A clean heart create in me, O God" (ver. 10). "Create"—he meant to say, "as it were begin something new." But, because repentant he was praying (that had committed some sin, which before he had committed, he was more innocent), after what manner he hath said "create" he showeth. "And a right spirit renew in my inner parts." By my doing, he saith, the uprightness of my spirit hath been made old and bowed. …

Exposition on Psalm 51 ↗

Bede

It is only by participation in the divine goodness that a rational creature is recognized as being capable of becoming good. Hence the Lord also bears witness by a benevolent promise that "your Father from heaven will give his good Spirit to those who ask him." This is to point out that those who of themselves are evil can become good through receiving the gift of the Spirit. …

Homilies on the Gospels 2:14 ↗

Thomas Aquinas

"A clean heart." Above, the Psalmist asked for sin to be removed; here he asks for the effects of sin to be removed, which are two: namely, the defilement of the soul and the disorder of the affections. The first effect occurs because a person is attached to earthly things; hence he asks for cleanness of heart. Mt. 5: "Blessed are the clean of heart, for they shall see God"; and therefore he says, "Create in me a clean heart, O God, and a right spirit." This cleanness of heart, God alone can restore. …

Exposition on the Psalms of David ↗