He shall send from heaven, and save me. David, as I have repeatedly had occasion to observe, interlaces his prayers with holy meditations for the comfort of his own soul, in which he contemplates his hopes as already realised in the event. In the words before us, he glories in the divine help with as much assurance as if he had already seen the hand of God interposed in his behalf. When it is said, he shall send from heaven, some consider the expression as elliptical, meaning that he would send his angels; but it seems rather to be an indefinite form of speech, signifying that the deliverance which David expected was one not of a common, but a signal and miraculous description. The expression denotes the greatness of the interposition which he looked for, and heaven is opposed to earthly or natural means of deliverance. What follows admits of being rendered in two different ways. We may supply the Hebrew preposition m, mem, and read, He shall save me from the reproach; or it might be better to understand the words appositively, He shall save me, to the reproach of him who swallows me up. [1] The latter expression might be rendered, from him who waits for me. His enemies gaped upon him in their eagerness to accomplish his destruction, and insidiously watched their opportunity; but God would deliver him, to their disgrace. He is said to strike his enemies with shame and reproach, when he disappoints their expectations. The deliverance which David anticipated was signal and miraculous; and he adds, that he looked for it entirely from the mercy and truth of God, which he represents here as the hands, so to speak, by which his assistance is extended to his people.

Footnotes

1 - In this all the ancient versions agree: They make chrph, chereph, a verb, and not a noun, regarding it as applicable to God, and conveying the idea that He would deliver David, having put to shame, or to reproach, his enemies. Thus, in the Septuagint, it is "edoken eis oneidos" and in the Vulgate, "dedit in opprobrium," "he gave to reproach;" and in like manner in the Chaldee, Syriac, Arabic, and Ethiopic versions.

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 Psalm 56:1. The idea here is, that God would "rebuke" or "reproach," to wit, by overthrowing him that sought to devour or destroy him. God had interposed formerly in his behalf Psalm 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 Psalm 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.

He shall send from heaven, and save me - Were there no human agents or earthly means that he could employ, he would send his angels from heaven to rescue me from my enemies. Or, He will give his command from heaven that this may be done on earth.
Selah - I think this word should be at the end of the verse.
God shall send forth his mercy and his truth - Here mercy and truth are personified. They are the messengers that God will send from heaven to save me. His mercy ever inclines him to help and save the distressed. This he has promised to do; and his truth binds him to fulfll the promises or engagements his mercy has made, both to saints and sinners.

He shall send from (d) heaven, and save me [from] the reproach of him that would swallow me up. Selah. God shall send forth his mercy and his truth.
(d) He would rather deliver me by a miracle, than that I should be overcome.

He shall send from heaven, and save me,.... His angel, as the Targum adds; or his angels, as Kimchi; who are ministering spirits, sent forth by him, to encamp about his people, and guard them, as they did Jacob when in fear of Esau, Genesis 32:1; or to deliver them out of trouble, as Peter when in prison, Acts 12:7; or rather the sense may be, that David did not expect any help and deliverance in an human way, by means of men on earth; but he expected it from above, from heaven, from God above, and which he believed he should have; and he might have a further view to the mission of Christ from heaven to save him, and all the Lord's people; and which he may mention, both for his own comfort, and for the strengthening of the faith of others in that important article;
from the reproach of him that would swallow me up. Meaning Saul; see Psalm 56:1. The Targum renders it,
"he hath reproached him that would swallow me up for ever;''
and to the same sense the Septuagint, Vulgate Latin, Ethiopic, Arabic, and Syriac versions; disappointed them, and filled them with reproach, shame, and confusion.
Selah; on this word; see Gill on Psalm 3:2.
God shall send forth his mercy and his truth; shall manifest and display the glory of these his perfections, his mercy and grace, his truth and faithfulness, in his deliverance and salvation; and which are remarkably glorified in salvation by Christ Jesus; and who himself may be called "his grace and his truth" (n), as the words may be rendered; he being the Word of his grace, and truth itself, and full of both; and by whom, when sent forth, grace and truth came, John 1:14; it may also intend a constant supply of grace, whereby God would show forth the truth of his promises to him.
(n) "gratiam et veritatem suam", Cocceius, Gejerus, Michaelis.

from . . . swallow me up--that pants in rage after me (Psalm 56:2).
mercy and . . . truth-- (Psalm 25:10; Psalm 36:5), as messengers (Psalm 43:3) sent to deliver him.

Send forth - Will discover them, by affording his gracious help in pursuance of his promises.