Christus Vincit is a rousing call and respomse type chant from the 8th century or thereabouts.

There are lots of different versions - even A New Book of Old Hymns has included two different versions over time:

2014 pdf excerpt - a Sarum one from the Christus Rex Ballarat to Bendigo Pilgrimage
2016 pdf excerpt - a more generic one from the Pius X Hymnbook by Percy Jones - actually, this was the first version I typed up, so much earlier copies of the hymnbook use this one too.

The first Mass of Pope Leo began with Christus Vincit:

Here is another variation by Clamavi De Profundis:

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