We no longer make the distinction between the three titles of inspired compositions and refer to all of them as simply the psalms. Sadly, this has resulted in the widespread belief that when the Bible says “psalms” it means the book of Psalms and when it says “hymns” and “spiritual songs” it means uninspired compositions.
When the writers of the New Testament used the terms translated in our English Bibles as “psalms,” “hymns” and “songs” they were referring to the inspired psalms. They were certainly not referring to uninspired compositions. Every first century believer, Jew or gentile, who heard “psalms, hymns and spiritual songs” knew immediately that the reference was to the inspired compositions in the psalter. That is how the compositions were titled. It would never occur to them to think that “psalms, hymns and spiritual songs” meant “inspired psalms and uninspired compositions.”