Exclusive Psalmody (EP) is a doctrine that has been adopted by some historically reformed churches. In brief, EP adherents claim that the 150 Psalms of the Old Testament (OT) are the only Biblically warranted songs allowed in worship. This supposedly aligns with the reformed Regulative Principle of Worship (RPW) that is summarized as "if commanded, required - if not commanded, prohibited".
The argument against EP and aligned to the RPW is as follows:
1. The Psalms are a trustworthy guide to proper worship.
2. The Psalms command that we sing of the works and deeds of the Lord:
[Psalm 9:11 ESV] Sing praises to the LORD, who sits enthroned in Zion! Tell among the peoples his deeds!
[Psalm 105:2 ESV] Sing to him, sing praises to him; tell of all his wondrous works!
[Psalm 107:22 ESV] And let them offer sacrifices of thanksgiving, and tell of his deeds in songs of joy!
3. The works and deeds of the Lord Jesus are most fully revealed in the New Testament.
4. The Psalms command new songs (Psalms 33:3, Psalms 40:3, Psalms 96:1, Psalms 98:1, Psalms 144:9, Psalms 149:1)
Therefore, new songs concerning the works and deeds of Jesus from the NT are commanded and required for proper worship.
Therefore, EP is an erroneous doctrine.
The New Testament is replete with the revealed and explicit works and deeds of Jesus that the Psalms only obscurely reference in "types and shadows", thus the church is commanded to sing new songs of this newly revealed truth. EP demands we exclude in sung worship the revealed works and deeds of Jesus as clearly revealed in the NT.
Remember, to invalidate the conclusions, you must first invalidate the premises or prove non sequitur.