Adam Clarke on Psalms 57:1

Be merciful unto me - To show David's deep earnestness, he repeats this twice; he was in great danger, surrounded by implacable enemies, and he knew that God alone could deliver him.

My soul trusteth in thee - I put my life into thy hand; and my immortal spirit knows no other portion than thyself.

In the shadow of thy wings - A metaphor taken from the brood of a hen taking shelter under her wings when they see a bird of prey; and there they continue to hide themselves till their evemy disappears. In a storm, or tempest of rain, the mother covers them with her wings to afford them shelter and defense. This the psalmist has particularly in view, as the following words show: "Until these calamities be overpast."