Today I learned that Hail Mary is usually capitalized.
Yesterday I told a co-worker he should capitalize the term, but then I realized I only believed it was a proper noun because it “felt” right. About 95% of my grammar, spelling, and punctuation is done by how it feels or sounds to me, so this isn’t unusual but if I’m going to correct people I should be sure! So today I looked it up.
Liturgical printing style guides have it capitalized, as did 9 out of 10 dictionaries I checked.