Augustine of Hippo

"Unless ye be converted," He says, "He will brandish His sword" [Psalm 7:12]. The Lord Man Himself may be taken to be God's double-edged sword, that is, His spear, which at His first coming He will not brandish, but hides as it were in the sheath of humiliation: but He will brandish it, when at the second coming to judge the quick and dead, in the manifest splendour of His glory, He shall flash light on His righteous ones, and terror on the ungodly. …

Exposition on Psalm 7 ↗

Jerome

'Unless ye be converted, he will brandish his sword.' This passage admits a twofold understanding. For many think it is said of God, so that, for example's sake, it is said to sinners. And what does it say? 'Unless ye be converted, he will brandish his sword': that is, unless ye do penance, the Lord will strike you with his sword. But let us see whether the things that follow can be fitted to the person of the Lord. 'He hath bent his bow, and made it ready.' This can be said of God. …

Homily 3, on Psalm 7 ↗

Thomas Aquinas

Then when he says, "Unless," he shows that the Lord prepares himself to inflict punishment, even though he delays for a reason. And this preparation is set forth: first on God's part as the one punishing; second on man's part as the one punished or receiving, at "Behold, he is in labor." The preparation on God's part is described according to man's preparation against merit or sin: because "the impious man and his impiety are hateful to God" (Wis. 14). …

Exposition on the Psalms of David ↗