Albert Barnes Commentary Psalms 57:3

"He will send from heaven, and save me, [When] he that would swallow me up reproacheth; Selah God will send forth his lovingkindness and his truth." — Psalms 57:3 (ASV)

He shall send from heaven - That is, from himself; or, he will interpose to save me. The psalmist does not say “how” he expected this interposition - whether by an angel, by a miracle, by tempest or storm, but he felt that help was to come from God alone, and he was sure that it would come.

And save me from the reproach... - This would be more correctly rendered, “He shall save me; he shall reproach him that would swallow me up.” So it is rendered in the margin. On the word rendered would swallow me up, see the notes at (Psalms 56:1). The idea here is that God would “rebuke” or “reproach,” namely, by overthrowing him who sought to devour or destroy him. God had interposed formerly in his behalf (Psalms 57:2), and he felt assured that he would do it again.

Selah - This seems here to be a mere musical pause. It has no connection with the sense. See the notes at (Psalms 3:2).

God shall send forth his mercy - In saving me. He will manifest his mercy.

And his truth - His fidelity to his promise; his faithfulness to those who put their trust in him. He will show himself true to all the promises which he has made. Compare Psalm 40:11.