Augustine of Hippo

"And He bowed the heaven, and came down" [Psalm 18:9]. And He humbled the just One, that He might descend to men's infirmity. "And darkness under His feet." And the ungodly, who savour of things earthly, in the darkness of their own malice, knew not Him: for the earth under His feet is as it were His footstool.

Exposition on Psalm 18 ↗

Jerome

Because we are little and lowly and unable to lift ourselves up to him, the Lord stoops down to us and in his compassionate kindness deigns to hear us. In fact, because we are people and cannot become gods, God became man and inclined himself, as it is written: "He inclined the heavens and came down."

HOMILY ON PSALM 114(116a)

Thomas Aquinas

"He inclined." Here he treats of the winds. Note that the matter of wind is a dry vapor or exhalation that is heated but not so refined that it can ascend to the highest place, nor so heated; hence it is impeded by the cold, thickened, and repelled downward. And this, repelled, moves the air. Nevertheless, it has enough warmth that it is not so overcome by the cold as to be converted to earth; and it is called haze, and is said to be "under his feet," because it is not as high as that which is kindled into flame. …

Exposition on the Psalms of David ↗