64:7-8a David's enemies had assailed him with words that they used as deadly arrows, but God would shoot them with His arrow of judgment. With it God would make them fall in battle. The NASB is a bit misleading in verse 8. The NIV is clearer. It reads, "He will turn their own tongues against them."
64:8b-10 David identified the reactions of two groups of people to God's judging his evil assailants. Those who observed the judgment would do two things. They would fear doing the same thing themselves and would declare to others what He did having considered it themselves. Second, the righteous would also have a double response. They would rejoice in God's will being done and they would renew their trust in the Lord.
The godly should commit their case to God in prayer when they become targets of malicious gossip. They can also rest in the assurance that God will eventually turn the antagonism of the wicked back on themselves (cf. 1 Sam. 25). He will do so for His own glory and for the welfare of those who trust in Him.122
