John Calvin Commentary Psalms 64:7

"But God will shoot at them; With an arrow suddenly shall they be wounded." — Psalms 64:7 (ASV)

And God shall shoot an arrow at them. The Psalmist now congratulates himself in the confident belief that his prayers have not been in vain but have already been answered. Although there was no outward sign of God’s approaching judgment, he declares that it would suddenly be executed; and in this, he offers a remarkable proof of his faith.

He saw the wicked growing hardened in their prosperity and presuming they would escape punishment due to divine forbearance and God’s apparent overlooking of their actions; but instead of yielding to discouragement, he was sustained by the belief that God, according to His usual way of acting towards the wicked, would visit them at an unexpected moment, when they were flattering themselves that they had escaped and were indulging in excessive confidence.

This is a truth that should comfort us when we are subjected to prolonged trials: that God, in delaying to punish the ungodly, does so with the express purpose of later inflicting more fitting and deserved judgments upon them, and, when they say, Peace and safety, overwhelming them with sudden destruction (Jeremiah 8:11).