John Calvin Commentary Isaiah 37:15

"And Hezekiah prayed unto Jehovah, saying," — Isaiah 37:15 (ASV)

Then Hezekiah prayed to God, saying, O Jehovah of hosts. Because Sennacherib was the agent employed by Satan to shake Hezekiah's faith, he defends himself with this rampart: that God possesses infinite power; for by bestowing on God those lofty praises, he undoubtedly encourages himself to confidence in supplication.

So that our prayers may not be unsuccessful, we ought always to hold it as certain that God is the rewarder of all who seek him (Hebrews 11:6). It was especially necessary for the pious king to believe that although wicked men mock and undervalue the power of God, it still remains undiminished, so that he might boldly and undauntedly remove the obstruction by which Satan had attempted to stop the progress of his confidence.

The heroic courage of the pious king appeared not only in his contending with a wicked king to maintain God's power, but also in his exalting it in his own heart and appealing to God as the witness of his inward feelings. Accordingly, before forming any prayer, he overturns the delusions by which Satan had endeavored to shake his courage.