Puritan Board Sophomore
But most commentators could be wrong, since the Bible does not say so directly. There are commentators who believe in fantastic rubbish like a secret rapture or an earthly millenial kingdom! There are commentators who fall on either side of the baptism debate. We cannot say something is sure because most commentators agree.These are some innovative views. It's known that the Hallel Psalms were sung as part of the Passover. Most commentators believe that "hymn" in Matthew certainly is referring to a Psalm of that collection.
And, the Psalms are mentioned as a collection, a book, in the NT including by Christ as he spoke to the Emmaus disciples. I think individual songs in the inspired collection are properly referred to as Psalms.
Regarding what is a psalm itself, who's to say that the word cannot now nor ever did refer to an uninspired ditty? A song? a tune? an ode? a hymn? an anthem? See Tim Foster's word study a few posts back.