John Calvin 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, then shall mine enemies turn back. Here he boasts of victory with even more confidence than before, specifying, as it were, the very moment when his enemies were to be turned back. He had no tangible evidence of their approaching destruction, but from the firm reliance he placed on the promise, he was able to anticipate the coming time and resolved to wait for it with patience.

Though God might not hurry to intervene and might not scatter his enemies the very instant he prayed, he was confident that his prayers would not be disappointed. His ground for believing this was simply a conviction of the truth that God never thwarts the prayers of His own children. With this conviction thoroughly fixed in his mind, he could moderate his anxieties and calmly await the outcome. It is instructive to notice that David, when he sought to secure the obtainment of his request, did not pray in a hesitating or uncertain spirit, but with a confident assurance of being heard. Having once reached this faith, he defies the devil and all the host of the ungodly.