John Calvin Commentary Psalms 40:2

"He brought me up also out of a horrible pit, out of the miry clay; And he set my feet upon a rock, and established my goings." — Psalms 40:2 (ASV)

And he drew me out of the roaring pit. Some translate this as from the pit of desolation, because the verb שאה (shaah), from which the noun שאום (shaon) is derived, signifies to destroy or to waste, as well as to resound or to echo. But it is more appropriate to consider that there is an allusion here to the deep gulfs, where the waters gush with a tumultuous force.

By this similitude he shows that he was placed in as imminent peril of death as if he had been cast into a deep pit, roaring with the impetuous rage of waters. To the same purpose also is the similitude of the miry clay, by which he intimates that he had been so nearly overwhelmed by the weight of his calamities, that it was no easy matter to extricate him from them.

Next, there follows a sudden and incredible change, by which he makes manifest to all the greatness of the grace which had been bestowed upon him. He declares that his feet were set upon a rock, whereas formerly he had been overwhelmed with water; and that his steps were established or upheld, whereas before they were not only unsteady and slippery, but were also stuck fast in the mire.