Albert Barnes Commentary Psalms 31:21

"Blessed be Jehovah; For he hath showed me his marvellous lovingkindness in a strong city." — Psalms 31:21 (ASV)

Blessed be the Lord - An expression of thanksgiving for the evidence that God had heard him in his troubles, and had answered him.

For he has shown me his marvelous kindness - Literally, “He has made his mercy wonderful;” that is, he has shown me such mercy as to be an object of admiration and astonishment. It was not ordinary kindness, such as is shown to people every day; it was so uncommon—so far beyond all expectation—so separate from second causes and the agency of man—so marked in its character—as to fill the mind with wonder.

In a strong city - Margin, “fenced city.” This may mean either that he had thus placed him literally in a strongly fortified city where he was safe from the fear of his enemies; or that he had interposed in his behalf, and had given him protection as if he had brought him into such a strongly fortified place. Jarchi supposes that the city of “Keilah” (1 Samuel 23:7) is here intended. But this is improbable. All that the passage necessarily implies is that God had given him protection as if he had been placed in a strongly fortified town where he would be safe from danger.