109:21-25 David asked the Lord to deal with him in harmony with His loyal love for the sake of God's reputation. David had sought to follow the Lord faithfully, and God had promised to bless people who did that. However, David was not experiencing God's blessing. This made other people question God's justice and faithfulness. If God would again bless David and curse his enemy, this would show onlookers that God's promises are trustworthy. In these verses David described how he felt in his downtrodden condition.
The Israelites usually practiced fasting (v. 24) for spiritual reasons rather than for physical reasons such as losing weight. They went without food and sometimes drink temporarily to spend that time in more important activity, specifically seeking God in prayer. Therefore we should probably understand David's reference to fasting as a reference to prayer. He had prayed earnestly about the situation this psalm reflects. His extended prayer and fasting had made him physically weak.
The people who reproached David (v. 25) were evidently his enemies. These are the other people in view throughout the psalm.
109:26-31 David called on Yahweh to save him from the distress in which he found himself in a way that would teach his enemies that God had delivered him. This would vindicate David and all he stood for in their sight. Again he asked God to shame his accusers and thereby signal divine disapproval of their opposition to God's righteous servant. David concluded with a confident assertion that God would indeed vindicate him. This would result in the psalmist thanking and praising the Lord.
Believers can pray for the vindication of righteousness with good precedent in the psalms. With the light of later revelation we understand better than David did that God will not always vindicate the godly in this life, but He will do so eventually (Deut. 32:35; Rom. 12:19; et al.; cf. Acts 17:30-31; Rev. 7:17; 21:4). In David's day and in ours God normally vindicates the righteous before they die, but His decision to postpone vindication often makes it appear that He is unjust (cf. Job). David's "bottom line"concern in this psalm was the vindication of God Himself (v. 31), but he also wanted relief from his oppressors.176
