John Gill Commentary Psalms 56:9

"Then shall mine enemies turn back in the day that I call: This I know, that God is for me." — Psalms 56:9 (ASV)

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.