Augustine of Hippo

"With the holy You shall be holy" [Psalm 18:25]. There is a hidden depth also, wherein You are known to be holy with the holy, for that You make holy. "And with the harmless You shall be harmless." For Thou harmest no man, but each one is bound by the bands of his own sins. [Proverbs 5:22]

Exposition on Psalm 18 ↗

Thomas Aquinas

Next the reason for retribution is set forth; therefore there follows, "With the holy one." Regarding this he does two things. First he sets forth the reason for retribution. Second, he expounds it, at "for you will save the humble people." The first two verses can be understood in two ways. In one way, as speaking to God; and thus the literal sense is, as if to say: You, O Lord, "with the holy one will be holy." And thus he says two things: namely that God is the rewarder and approver of the good. …

Exposition on the Psalms of David ↗

Bernard of Clairvaux

The love of God begets the love of the soul, and his preceding intention makes the soul intent, and his solicitude makes it solicitous. For I know not by what closeness of nature, once the soul shall have been able to behold the glory of God with face unveiled, it is necessary that it soon be conformed to it and transformed into the same image. Therefore, as you shall have prepared yourself for God, so it is necessary that God appear to you. …

Sermons on the Song of Songs, Sermon 69 ↗