Where to start in sacred music?
First thought: This depends from your preferences with classical music.
If you're into baroque music, Messiah or Christmas Oratorio could be obvious recommendations. But Monteverdi's Vespers and Biber's Missa Salisburgensis (= the obvious predecessor of Mahler 8

) could be worth a try.
If Beethoven is your favorite, the Missa Solemnis. (The Mass in C major isn't too bad.)
Etc. ... Mendelssohn -> St. Paul, Elijah. But "Lobgesang"/"Hymn of Praise" could do the job.
Bruckner -> Te Deum and Psalm 150. Maybe Mass No. 3 F minor.
Brahms -> Deutsches Requiem
French Repertoire: Faure, Requiem; Duruflé, Requiem (charming!); Poulenc: Gloria
Bach cantatas are varied in styles and individual pieces in numbers, so probably no one can be sure which to recommend. Me too, I would always recommend large scale pieces like masses or Te Deum of Marc Charpentier.
Oh, that's no so difficult. Just listen to all cantatas several times, and you have a good overview.
Early: BWV 131, BWV 106 ("Actus tragicus")
Weimar: BWV 61, BWV 21, BWV 63
Leipzig I: BWV 22/23, BWV 69a, BWV 95, BWV 65
Leipzig II: BWV 2, BWV 93, BWV 180, BWV 127
Leipzig III: BWV 56, BWV 98, BWV 79
Later, including "Ratswahl": BWV 80, BWV 51, BWV 29, BWV 82, BWV 140