Psalm 12 is one in which the believer testifies that the Word of God is trustworthy and true, as opposed to the lies of men. This would be a good song to sing before your sermon, perhaps before reading from God’s Word.
o Free Church of Scotland (lyrics) (audio)
There are many Psalms that draw attention to vows and keeping the commitments that one has made (Psalm 22; Psalm 50; Psalm 56; Psalm 61; Psalm 65; Psalm 66; Psalm 76; Psalm 116). Many of these Psalms would also be appropriate for singing after your sermon. Psalm 56 or Psalm 116 might be especially relevant since the Psalmist commits to keeping vows/promises in response to God’s work of salvation.
o Seedbed Psalter (56)
o Psalm 56: Reformed Churches of New Zealand (score and lyrics) (audio)
o Psalm 116: Reformed Churches of New Zealand (score and lyrics) (audio)
33 “Again you have heard that it was said to those of old, ‘You shall not swear falsely, but shall perform to the Lord what you have sworn.’