I agree that the RPW demands that we ought to sing Scripture however it would be wrong, I believe, to argue that we should therefore sing all of Scripture. I can’t imagine you are advocating singing 1 Chronicles 1-9 for instance!!
We are to sing songs, and the RPW commands us to sing
inspired songs. So we are to limit our singing to Scripture but only that Scripture that is to be sung (if that makes any sense!). The statement “we ought to sing Scripture” is too broad, for whilst all inspired songs are Scripture, not all Scripture are inspired songs. Therefore our position ought be to say that the RPW demands we sing
inspired songs only.
What then are inspired songs? Well we have OT songs including Exodus 15 and Judges 5. Also the Psalms. Are there any NT songs? I see no biblical evidence of NT songs: “And Mary
said, My soul doth magnify the Lord,” (Luke 1:46); “And his father Zacharias was filled with the Holy Ghost, and prophesied,
saying,” (Luke 1:67); and, “Then took he him up in his arms, and blessed God, and
said,” (Luke 2:28). These then were said, not sung.
The question then is, does God want us to sing all inspired songs? The answer is a fairly simple no. It is highly doubtful that the songs in the OT were ever sung more than once (there are one or two exceptions but then these are commanded to be sung from generation to generation). However the Spirit has composed a Book of Praises or Book of Hymns or Book of Psalms that is made up of psalms, hymns and songs. We have then been told to sing “psalms”, “hymns” and “songs”. Hence I would argue that the exclusive use of the psalter is biblical.
