One of the great challenges in the history of singing by the people of God, in a way not confined to professionals or choirs or other select (typically male) voices, is the particular challenge of what was permissible to sing. It ended up depending on which part of the Reformation you found yourself living in.
Martin Luther was a pretty nasty person in some ways; his bitter animus against Judaism can't be ignored or brushed off as his being "a person of his time," having done too much damage by giving aid and comfort to antisemitic hatemongers for literally centuries now (and yes, the likes of Hitler would be included in that). For the subject at hand here, on the other hand, it can't be avoided that Luther was a major force for good in opening up the song of the church to all the people of the church. The creation of hymns as a song for use in the service of worship first flourished in the churches that followed after the Lutheran Reformation tradition. Indeed, Luther himself did contribute (wittingly or not) one of the major contributions to the early corpus of the chorale, the term applied to congregational songs in that early Lutheran practice.
On the other hand, Ulrich Zwingli, the Swiss reformer, was less of an encourager of music, although some scholars will contend that he was not the virulent opponent of music in the church that history has made him out to be. He was perfectly o.k. with busting up church organs, however, so he can't be a real good guy.
As is usually the case in these things, John Calvin ends up somewhere in the middle. He wasn't too much of a musician himself, but on a human basis (not unlike Augustine) he was fond enough of music to be suspicious of it. His complicated and evolving attitude about music was covered in a previous blog entry; for the moment the idea of interest is that churches of Calvinist background did not sing the kind of texts we would call "hymns," as in texts not from scripture. Texts from the Psalms were the primary stuff of singing, with a few other biblical texts, or canticles, thrown in as well. Mostly psalms, though.
This restriction doesn't endure forever. It is not too many years before churches under the Reformed tradition are starting to expand their repertoire to include what was called in the language of the time "hymns of human composure." The way was somewhat smoothed by the work of Isaac Watts and others, who wrote translations and paraphrases of psalm texts that were "Christianized," or filled out with references to the Messiah as Christians understood the term; where Christian reading and interpretation concluded that a psalm was referring to Christ, Watts went ahead and made the reference explicit.
Even those who were eager for such hymn texts could recognize that Watts was doing something different, though, and that phrase "hymns of human composure" gets tossed around by old-guard types as something that Christians should not be singing in worship.
Now I'm hoping you're seeing several reasons that this particular phrase - "hymns of human composure" - sounds a little strange or even downright weird as a reason to forbid congregational singing. For one, even if the hymns aren't of "human composure," at least one other thing in the service is going to be so humanly composed: your sermon. I mean, I know you're relying upon the Holy Spirit in your exegesis and study and preparation and writing and all that, but ultimately you're still the one who has to put it all together.
Another point of "wait, what?"-ness about this phrase is that, yes, the psalms themselves are of "human composure." Now here you get into potential conflicts about the interpretation of scripture and indeed the very nature of the writing down of scripture, but hopefully the handful of folks who actually read this blog aren't shocked by the idea that human beings actually wrote down the poetic texts that were collected into the book of the Bible we call Psalms. David may have created some of them (probably had a servant or someone else write them down, though); a Temple figure named Asaph appears to have written some; and there are a lot of them whose authorship is simply unknown. But people wrote the psalms in Psalms. So, to put it bluntly, psalms are "hymns of human composure."
Finally, exactly why is a "hymn of human composure" bad?
Hymns can do many things. A hymn of praise offers, well, praise to God. This is, of course, something we are instructed to do in scripture - especially a lot of those psalms that begin or end with the point-blank exhortation "Praise the Lord!" A psalm that teaches falls right in line with those oft-quoted instructions from Colossians 3:16 to "teach and admonish one another." The making and singing of hymns becomes an act of obedience to God, fulfilling not only what God calls us to do but what we need to do, what we must do because of the Spirit moving and leading within us, placing within us not merely the call but the urgent desire to worship God. In other words, inspiring us - even though we don't live in the era of the Psalms. Yes, it's true: divine inspiration didn't stop at the closing of the biblical canon, and that hymn has as at least as much claim to divine inspiration as your sermon.
Hymns of human composure? They're the best kind.
David was human, you know (sometimes painfully so).