Since I consider sacred music composition one of the main aspects of my career, perhaps you would allow me to contribute my two cents. This has been my experience and tack over the course of my life as a composer.

1. Listen to music
2. Analyze music (look at scores while listening if possible)
3. Play music (piano or organ) to best understand music theory. This is critical!
4. Learn theory (traditional harmony, counterpoint) (AVOID avante garde which is based in chaos, the opposite of order)
5. Do excersizes (four part harmonizations, counterpoint)
6. Compose (at first imitating the styles past)
7. Repeat this process daily if possible, and NEVER stop learning and evaluating and listening to familiar and unfamiliar music.

During this time you will discover your personal style. (This is the God given gift that you have that is your autography. In other words, when you write a letter or note to someone on paper, your handwriting is unique in the way you shape your letters, etc. The same goes for music and art. You ALREADY HAVE a style, which will emerge in your compositional style.

NOTE: My personal take is that once you hit the classical era and beyond, you are wandering away from the technique and the art of sacred music and more into operatic, symphonic and dramatic styles that don't really lend themselves to sacred music. The music becomes the end of the means and becomes opaque and measured and even dated. Sacred music is elusive, mystical, timeless and transparent.

LIke one TV commercial says, "Eat, Live and Breathe this Stuff". That would seem to be true about mastery over anything, and sacred music is no exception.