31 Nevertheless, God, full of mercy, forsook them not. “And He saw when they were in adversity, when He heard their complaint”. “And He thought upon His covenant, and repented, according to the multitude of His mercies”. He says, “He repented,” because He changed that wherewith He seemed about to destroy them. With God indeed all things are arranged and fixed; and when He seems to act upon sudden motive, He does nothing but what He foreknew that He should do from eternity; but in the temporal changes of creation, which He rules wonderfully, He, without any temporal change in Himself, is said to do by a sudden act of will what in the ordained causes of events He has arranged in the unchangeableness of His most secret counsel, according to which He does everything according to defined seasons, doing the present, and having already done the future.
And who is capable of comprehending these things? Let us therefore hear the Scripture, speaking high things humbly, giving food for the nourishment of children, and proposing subjects for the research of the older: that everlasting covenant “which He made with Abraham,” not the old which is abolished, but the new which is hidden even in the old. “And pitied them,” etc. He did that which He had covenanted, but He had foreknown that He would yield this to them when they prayed in their adversity; since even their very prayer, when it was not uttered, but was still to be uttered, undoubtedly was known unto God.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)