In the first part of Psalm 18, the Messiah expressed his confident trust in God (Psalm 18:1–2) before asking the LORD to deliver him from his enemies in a time of great distress (Psalm 18:3—6). He then described the LORD’s powerful intervention as God the Warrior causes the earth to heave and the seas to dry up (Psalm 18:7–15). The next section—Psalm 18:16—24—testifies to the deliverance of the Messiah. God’s King is delivered from his enemies because he is righteous. There is no sin to be found in him, for he has been completely obedient to God’s commandments. With these statements, we realise that this song is ultimately about the Lord Jesus Christ. He alone has, by virtue of his sinless nature, a heart of perfect righteousness. By his Spirit our Lord inspired David to compose this song to show us his perfections, affirming to us that his prayers will always be heard by God, thereby giving us confidence to pray in his name.
Given the text’s focus on the righteousness of the Messiah, Psalm 18:16—24 would be appropriate for singing either before or after your sermon.
Another psalm that may be appropriate for singing is Psalm 34. According to its introduction, it was written after David’s first sortie into Philistia (cf. 1 Samuel 21). In the New Testament, the apostle Peter quotes Psalm 34:8 when concluding his call for Christians to be holy (cf. 1 Peter 1:15—2:3). In his quotation he identifies the LORD with Jesus, once again showing us how the apostles understood the Old Testament Scriptures. He also quotes from Psalm 34:12–16 (cf. 1 Peter 3:10–13) in order to encourage Christians to pursue righteous living.
Given Psalm 34’s focus on godly living, this would be most appropriate for singing after your sermon.
1 Now the Philistines had gathered all their forces at Aphek. And the Israelites were encamped by the spring that is in Jezreel.