Some thing to remember about Medieval music is the influence of the Gregorian chant. The medieval mass was primarily used as a reminder of the sacrifice Jesus made on the cross; there were no liturgical movements like the Kyrie or Agnus Dei as we see in Renaissance masses.
During the early Renaissance period there was a surge of secular songs (madrigals, secular motets, instrumental pieces not used in mass) due to the rapiddly forming musical notation system - thanks to Guido of Arezzo and the late medieval monks. The church sought to bring in more people as is mission and one way they attempted to do so was through the use of secular music use in mass, but the lyrics were changed in order to fit the mass or other service.
Here are some links to check out as quotable sources:
Feel free to check the sources on this Wikipedia page: https://en.m.wikipedia.org/wiki/Mass_(music)