One way to follow tradition in hymnody while also enriching it lyrically with commentary: troping. The kind of troping that I mean is (1) the addition of new text to a melisma, i.e. to a single syllable of text sung on several different notes in succession, or (2) a new verse or verses, consisting of both text and music, between phrases of the original text. This is especially good, I think, for the Kyrie and the Agnus Dei, on account of their simple, biblically-based words and their great antiquity.
The linguistic benefit of troping is that interrelated streams of words can interact, the one commenting on the meaning and significance of the other. Especially when the two overlap in time, the text as a whole suggests interrelation, even more so than with conventional poetic parallelism. With two totally unknown segments of text, when hearers are totally unfamiliar with both the main text and the entwined comment text, which augments and elucidates it, this overlap is more difficult, but when one is as superficially simple as ‘Lord, have mercy; Christ, have mercy; Lord, have mercy,’ the troping week after week and the variation season by season can be quite useful.
Musically, planned troping is an opportunity for creativity in the writing of harmonies on the cantus firmus, a preëxisting melody forming the basis of a polyphonic composition. This can help foster the making of new music for the glory of God with more than just chordal harmony.
In CCM, one example of simple, two-part polyphony is ‘You Are Holy (Prince of Peace)’, but the troping of older texts can help a congregation to learn their significance without hearing an obtrusive prosaic explanation, thus both teaching something more catholic (i.e. more universal) in worship and affording opportunities for making something new. In my opinion, this is a sure win-win for the Church.
Clearly, a listener (presumably singing the cantus firmus or a familiar trope) can’t follow four independent lines of text, but troping and simpler polyphony can usefully add verbal and musical commentary, weaving these textures into the text to enhance the expression of both truth and beauty. They’re both a logical extension, moreover, of the call-and-response style of the Psalms, which are the starting point of the Church’s hymnody.