John Calvin Commentary Psalms 18:15

"Then the channels of waters appeared, And the foundations of the world were laid bare, At thy rebuke, O Jehovah, At the blast of the breath of thy nostrils." — Psalms 18:15 (ASV)

And the sources of the waters were seen. In this verse, David undoubtedly alludes to the miracle that was performed when the chosen tribes passed through the Red Sea. I have previously explained the purpose for which he does this. Since all the special benefits that God in ancient times bestowed upon any of the children of Abraham as individuals were so many testimonies by which He reminded them of the covenant He had once made with the whole people—to assure them that He would always continue His grace toward them, and that one deliverance might be for them a token or pledge of their perpetual safety and of God's protection—David appropriately connects the assistance God had sent from heaven to him in particular with that ancient deliverance of the Church.

Since the grace that he declares God had shown toward him was not to be separated from that first deliverance, as it was, so to speak, a part and an appendage of it, he beholds, as it were, at a glance or in an instant, both the ancient miracle of the drying up of the Red Sea and the assistance God granted to himself.

In short, God, who once opened a way for his people through the Red Sea and then showed himself to be their protector on the condition that they should be assured of being always maintained and preserved under his care, now again displayed his wonderful power in the defense and preservation of one man, to renew the memory of that ancient history.

From this it appears even more evidently that David, in using these apparently strange and exaggerated hyperboles, does not recite to us mere romantic fictions to please the imagination, in the style of pagan poets, but observes the style and manner that God had, as it were, prescribed to his people.

At the same time, we should carefully note the reason already mentioned, which compelled him to magnify God's grace with such splendid imagery: namely, because most people never seriously considered God's grace but, either through wickedness or stupidity, passed over it with eyes shut.

The Hebrew word אפיקים, aphikim, which I have translated as sources, properly signifies the channels of rivers; David, however, in this passage, evidently means that the very springs or sources of the waters were exposed, and that thus it could be discerned from where the great and inexhaustible abundance of water proceeds—the water that supplies the rivers and by which they always continue to flow in their course.