My main point was that, whatever might be your position, I myself cannot conceive of such a possibility that the readers in Paul's day would have understood the words, "psalms, hymns, and spiritual songs" to be understood as a reference exclusively to the Psalms. A reference to the Psalms? I can imagine that, yes. But exclusively to the Psalms? That comes completely out of left field for me.
Sorry for the delayed reply, John. I had a little mishap on my newly-acquired-and-soon-to-be-sold motorcycle on Saturday and I shattered the knuckle of my left thumb. Had surgery yesterday with two pins installed to hold the bone in place while it heals. God was very merciful and my injuries are relatively minor.
My apologies, too, for posting an argument that is already well-known to you and the PB, but since I'm new here, I didn't want to take anything for granted. Your reply, however, leads me to wonder just how deeply you have considered the exegesis of Paul's use of the triad "psalms, hymns and songs spiritual" in Colossians and Ephesians.
You say that the idea that Paul's readers would have read his words as a reference to the Psalms exclusively is inconceivable. ("You keep using that word. I do not think it means what you think it means.")
What leads me to my conclusion is not simply the fact that the three terms used by Paul were all commonly-known Psalter headings in the Septuagint version of the Psalms, but this fact, coupled with the force of the modifiers with which Paul surrounds them, as if to leave them no other option than to understand his reference as pointing them to the canonical Psalms.
"Let the word of Christ dwell in you richly in all wisdom; teaching and admonishing one another in psalms and hymns and spiritual songs, singing with grace in your hearts to the Lord."
First, he admonishes them to let the word of Christ dwell in them richly. In the context of songs of praise, as they were known to the apostle's readers, it is more than conceivable that they would think of the Psalms of David as being, in very fact, the Word of Christ, and that in a manner exclusive to the canonical catalogue of worship songs. To what other extant body of songs could he possibly have been referring? The very words of the Psalms were continually on our Savior's lips - He identified Himself in them to His disciples following His resurrection - and the inspired writers of the NT (particularly the author of Hebrews) often quote from the Psalter with the introduction "He said..." The reference to "the Word of Christ" combined with the mention of "psalms, hymns and songs spiritual" is a strong indicator of the apostle's intended meaning.
Add to this the modifier "in all wisdom," which adds force to the idea that the content Paul has in view is not man-made, but divinely inspired, for we can only be confident that we are admonishing "in all wisdom" when we exhort from the Word of Christ.
Now, having anchored his instruction in terms that point unmistakably to the Word of Christ, Paul proceeds to identify the particular matter with which we are to teach and admonish one another in song. His use of the words "teaching and admonishing" again direct us to Scripture, since it alone is God-breathed and profitable for such tasks. And the particular portion of God's Word to which he points us is that section delineated as "psalms, hymns, and songs" - terms which his readers, now especially being pre-disposed to think of "the Word of Christ," would have recognized as references to the Septuagint headings in the Psalter.
But the matter is put out of all doubt by Paul's immediate use of the additional modifier, "spiritual," which indicates that the psalms and hymns and songs, which are the very word of Christ and which are to be employed in mutual teaching and admonition, are not mere human compositions but are given by direct inspiration of the Holy Spirit.
There was only one body of songs known by Paul's readers that would fit all of the criteria he mentions here - and it was the Psalter.