In the western church for well over a millennium, the historic shape of the liturgy has encompassed a number of elements deemed essential to its proper celebration. Together these have formed the ordinary of the mass, including in outline form:
The Gloria in Excelsis
The Scripture Lessons
The Sursum Corda
The Eucharistic Prayer
The Agnus Dei
The Church of England’s Book of Common Prayer retained much of this shape of the liturgy; however, under the influence of the continental reformers, it moved a reading of the Decalogue to the beginning of the liturgy and moved the Gloria in Excelsis to the end, where it became a post-communion thanksgiving hymn. The Lutheran churches retained this structure as well, although only the ante-communion segment was used on Sundays when communion was not celebrated.
The Reformed and Presbyterian churches undertook a more radical reform, virtually eliminating the ordinary of the mass and substituting for it a liturgy which, while generally following the same order, nevertheless abandoned the historic hymns which had once made it up.
It has long seemed to me that, in so doing, the non-Lutheran reformers were doing more than just to reform; they, and more particularly their successors, came close to creating a new liturgy one that would inevitably seal the 16th-century breach within western Christendom. Had they taken a more measured approach, namely, to remedy the defects while preserving what was right and good, they might have seen fit to keep much of what we know as the ordinary of the mass.
Imagine, if you will, an alternative history in which Reformed Christians have grown up singing and loving the Gloria in Excelsis, knowing the Sanctus by heart, praying with heartfelt passion the Agnus Dei, and seeing in these hymns a liturgical treasure shared with all other Christians in the western tradition. There would be one less cause of division among these traditions, even where genuine confessional differences remained, because we would all hold in common something very beautiful and ancient a way of worshipping God in spirit and truth.