When I cry, then shall mine enemies turn back. Here he boasts of victory with even more confidence than formerly, specifying, as it were, the very moment of time when his enemies were to be turned back. He had no sensible evidence of their approaching destruction but from the firm reliance which he exercised upon the promise, he was able to anticipate the coming period, and resolved to wait for it with patience. Though God might make no haste to interpose, and might not scatter his enemies at the very instant when he prayed, he was confident that his prayers would not be disappointed: and his ground for believing this was just a conviction of the truth, that God never frustrates the prayers of his own children. With this conviction thoroughly fixed in his mind, he could moderate his anxieties, and calmly await the issue. It is instructive to notice, that David, when he would secure the obtainment of his request, does not pray in a hesitating or uncertain spirit, but with a confident assurance of his being heard. Having once reached this faith, he sets at defiance the devil and all the host of the ungodly.

When I cry unto thee - This expresses strong confidence in prayer. The psalmist felt that he had only to cry unto God, to secure the overthrow of his enemies. God had all power, and his power would be put forth in answer to prayer.
Then shall mine enemies turn back - Then shall they cease to pursue and persecute me. He did not doubt that this would be the ultimate result - that this blessing would be conferred, though it might be delayed, and though his faith and patience might be greatly tried.
For God is for me - He is on my side; and he is with me in my wanderings. Compare the notes at Romans 8:31.

When I cry unto thee, then shall mine enemies turn back - As soon as they know that I call upon thee, then, knowing that thou wilt hear and save, my enemies will immediately take flight. The cry of faith and prayer to God is more dreadful to our spiritual foes than the war-whoop of the Indian is to his surprised brother savages.
This I know - I have often had experience of the Divine interposition; and I know it will be so now, for God is with me. He who has God With him need not fear the face of any adversary.

When I cry unto thee,.... In prayer;
then shall mine enemies turn back; great is the strength of prayer; the effectual fervent prayer of the righteous avails much against their enemies: when Moses lifted up his hands, Israel prevailed: the cases of Asa, Jehoshaphat, and Hezekiah, prove it; this David was assured of, and knew it to be true by experience, his prayer being often the prayer of faith in this respect;
this I know: for God is for me; he knew that when he prayed his enemies would flee; because God was on his side, who is greater than they; or by this he knew that God was for him, and was his God, by hearing his prayers, and causing his enemies to turn back: or, however, let things go how they will, this he was assured of, that he had a covenant interest in God, and who would be his God and guide even unto death.

God is for me--or, "on my side" (Psalm 118:6; Psalm 124:1-2); hence he is sure of the repulse of his foes.