"God" is capitalized for the same reason "Earth" or "the Universe" is capitalized. What precisely is meant may differ, but it's rich even for a materialist to argue that the name for the personification of the ground of being shouldn't be capitalized (https://en.wikipedia.org/wiki/Conceptions_of_God)

In 2011, Pope Benedict XVI appointed Pärt a member of the Pontifical Council for Culture.

These two composers certainly meant God.