John Calvin 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 Jehovah! The Psalmist here proceeds to apply these general truths to his own circumstances, declaring that the goodness of God in preserving his life was wondrously displayed. As he speaks of aid that was suddenly and unexpectedly afforded him in very desperate circumstances, those interpreters judge rightly who here supply as, the mark of similitude. In this way, it was as in a fortified city that David lay open to every blow and had been exposed to every sort of injury; and he boasts that in his nakedness and destitution, the assistance of God had been of greater service to him than a well-fortified city or an impregnable fortress would have been.