10 “The Lord takes up the gentle”. For example; you understand not, you fail to understand, canst not attain: honour God's Scripture, honour God's Word, though it be not plain: in reverence wait for understanding. Be not wanton to accuse either the obscurity or seeming contradiction of Scripture. There is nothing in it contradictory: somewhat there is which is obscure, not in order that it may be denied you, but that it may exercise him that shall afterward receive it.
When then it is obscure, that is the Physician's doing, that you may knock. He willed that you should be exercised in knocking; He willed it, that He might open to you when you knock. By knocking you shall be exercised; exercised, you shall be enlarged; enlarged, you shall contain what is given. Be not then indignant for that it is shut; be mild, be gentle. Kick not against what is dark, nor say, It were better said, if it were said thus. For how can you thus say, or judge how it is expedient it be said?
It is said as it is expedient it be said. Let not the sick man seek to amend his remedies: the Physician knows how to temper them: believe Him who cares for you. Therefore what comes next?...“The Lord takes up the gentle, but humbles the sinners even to the ground,” he intended a certain sort of sinners to be understood, from the gentleness mentioned first. By sinners then in this place, we understand the fierce, and those who are not gentle. Wherefore does He “humble them even to the earth”? They carp at objects of understanding, they shall perceive only things earthly.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)