John Calvin Commentary Psalms 21:12

"For thou wilt make them turn their back; Thou wilt make ready with thy bowstrings against their face." — Psalms 21:12 (ASV)

For thou wilt set them as a butt. Since the Hebrew word שכם, shekem, which we have translated as a butt, properly signifies a shoulder, some understand it in that sense here. They explain the sentence this way: Their heads will be struck with heavy blows, so that with their bodies bent, their shoulders will appear to stick out. According to these interpreters, the subjugation of God's enemies is metaphorically indicated here.

However, there is another explanation more widely accepted, even among Jewish commentators: namely, that God will confine them to some corner and keep them from causing harm. They adopt this view because the Hebrew word שכם, shekem, is often used to denote a corner, quarter, or place.

Nevertheless, since the sacred writer, in the immediately following clause, portrays God as equipped with a bow, ready to shoot his arrows directly in their faces, I have no doubt that, continuing his metaphor, he compares them to a target, or a mound of earth on which it is customary to place the mark that is aimed at. The meaning, then, flows very naturally in this way: Lord, you will make them, as it were, a target against which to shoot your arrows.

The primary objective the Psalmist has in view is undoubtedly to teach us to exercise patience until God, at the opportune time, brings the ungodly to their end.