9 But how shall he decline, and fall? “For he has said in his heart, God has forgotten; He turns away His face, that He see not unto the end”. This is declining, and the most wretched fall, while the mind of a man prospers as it were in its iniquities, and thinks that it is spared; when it is being blinded, and kept for an extreme and timely vengeance: of which the Psalmist now speaks: “Arise, O Lord God, let Your hand be exalted”: that is, let Your power be made manifest.
Now he had said above, “Arise, O Lord, let not man prevail, let the heathen be judged in Your sight:” that is, in secret, where God alone sees. This comes to pass when the ungodly have arrived at what seems great happiness to men: over whom is placed a lawgiver, such as they had deserved to have, of whom it is said, “Place a lawgiver over them, O Lord, let the heathen know that they are men.” But now after that hidden punishment and vengeance it is said, “Arise, O Lord God, let Your hand be exalted;” not of course in secret, but now in glory most manifest.
“That You forget not the poor unto the end;” that is, as the ungodly think, who say, “God has forgotten, He turns away His face, that He should not see unto the end.” Now they deny that God sees unto the end, who say that He cares not for things human and earthly, for the earth is as it were the end of things; in that it is the last element, in which men labour in most orderly sort, but they cannot see the order of their labours, which specially belongs to the hidden things of the Son. The Church then labouring in such times, like a ship in great waves and tempests, awakes the Lord as if He were sleeping, that He should command the winds, and calm should be restored. He says therefore, “Arise, O Lord God, let Your hand be exalted, that You forget not the poor unto the end.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)