Renaissance polyphony flourished in 15th and 16th century Europe with some notable composers being:
Dunstable, John (c1390-1453) British
Dufay, Guillaume (c1398-1474) Franco-Flemish
Obrecht, Jacob (c1450-1505) Franco-Flemish Missa de Sancto Donatiano
Desprez, Josquin [des Prés, etc.] (c1440-1521) Franco-Flemish
Taverner, John (c1490-1545) British
Tallis, Thomas (c1505-1585) British
Lasso, Orlando di (1532-94) Franco-Flemish
Palestrina, Giovanni Pierluigi da (1525/6 - 1594) Italian
Victoria, Tomás Luis de (c1548-1611) Italian-Spanish
Byrd, William (1543-1623) British
Shakespeare, William (1564-1616) British
Monteverdi, Claudio (1567-1643) Italian
Besides the Obrecht cited above, I recommend
Palestrina: Masses & Motets;
Victoria: O Magnum Mysterium & Ascendens Christus in Altum;
The Essential Tallis Scholars;
Cantores in Ecclesia In Rome,
and some Gregorian Chant.
Sacred polyphony is both the artistic flowering of a long tradition of plain chant and also a wall of sorts, protecting the core music of the liturgy, gregorian chant, from secular enroachment.
The linked album of Gregorian chant, by Cisterian monks in Vienna, Austria, concludes with a Christmas mass:
16. Christmas Mass “Puer Natus”: Introitus “Puer Natus”
17. Christmas Mass “Puer Natus”: Kyrie “Missa De Angelis”
18. Christmas Mass “Puer Natus”: Gloria “Missa De Angelis”
19. Christmas Mass “Puer Natus”: Graduale “Viderunt Omnes”
20. Christmas Mass “Puer Natus”: Alleluia “Dies Sanctificatus”
21. Christmas Mass “Puer Natus”: Credo
22. Christmas Mass “Puer Natus”: Offertorium “Tui Sunt”
23. Christmas Mass “Puer Natus”: Sanctus “Missa De Angelis”
24. Christmas Mass “Puer Natus”: Agnus Dei “Missa De Angelis”
25. Christmas Mass “Puer Natus”: Communio “Viderunt Omnes”
The sung mass has parts that are the same through the year: the Ordinaries (Kyrie, Gloria, Credo, Sanctus, Agnus Dei) sung by the congregation, which also comprise the musical “Mass” artistic form, and parts that vary based on the season: the Propers (Introit, Gradual & Tract, Offertory & Communion), sung by the schola (choir). The Catholic encyclopedia explains that: Gradual, in English often called Grail, is the oldest and most important of the four chants that make up the choir’s part of the Proper of the Mass. Whereas the three others (Introit, Offertory, and Communion) were introduced later, to fill up the time while something was being done, the Gradual (with its supplement, the Tract or Alleluia) represents the singing of psalms alternating with readings from the Bible, a custom that is as old as these readings themselves.