Ralph Vaughan Williams - I first encountered his work singing in Notre Dame's Women's Liturgical Choir. His beautiful work has a very moving, haunting quality.
While doing a little research, I was very surprised to discover that he was an agnostic. I had figured he was a High Church Anglican given the impassioned character of his sacred music.
In 1921 he composed his Mass in G minor. Dedicated to his friend and fellow fantastic composer Gustav Holst. It was the first distinctively English Mass to be written since the 16th century. According to the almighty Wikipedia:
"Though the first performance was in a concert venue, Vaughan Williams intended the mass to actually be used in a liturgical setting." Interesting intent for an agnostic composer.
It is a beautiful Mass setting.
Kyrie - Mass in G Minor - R. Vaughan Williams