John Calvin Commentary Psalms 68:21

"But God will smite through the head of his enemies, The hairy scalp of such a one as goeth on still in his guiltiness." — Psalms 68:21 (ASV)

Surely God shall wound, etc. The enemies of the Church are fierce and formidable, and it is impossible for her to be preserved from their continued assaults without a vigorous protection being extended. To persuade us that she enjoys such a defense, David represents God as armed with dreadful power for the overthrow of the ungodly.

The verse is connected in scope with the preceding one, and we might translate the Hebrew particle אך, ach, as therefore, or for this reason; but it seems better to consider it as expressing simple affirmation. We should note that God counts all those as his enemies who unjustly persecute the righteous, and thus assures us of his being always ready to interpose for our defense.

The concern he feels for our preservation is forcibly conveyed by the expressions that follow, that he will wound the head of his enemies, and the crown of their hair; intimating that he will inflict a deadly and incurable wound upon those who harass his Church. This is still more strikingly brought out in what is added immediately afterwards, when God is described as wading through destruction.