Two elements are at play here. The first is the grammatic issue of proper noun versus regular noun. In general, "God" with a capital G is a proper noun, whereas "god" is not. This is very similar to "mom" or "dad". ("Did Mom say not to eat any cookies?" vs. "Your mom said not to eat the cookies.")

The second influence comes from a Christian tradition of capitalizing the "G" any time the god in question is the God of the Bible, even when "god" is a regular noun in the sentence. This same practice is often used with personal pronouns referencing God as well ("You", "He"). This is not a hard and fast rule of grammar, though it is a common practice, particularly among Christians.

Two elements are at play here. The first is the grammatic issue of proper noun versus regular noun. In general, "God" with a capital G is a proper noun, whereas "god" is not. This is very similar to "mom" or "dad". ("Did Mom say not to eat any cookies?" vs. "Your mom said not to eat the cookies.")

The second influence comes from a Christian tradition of capitalizing the "G" any time the god in question is the God of the Bible, even when "god" is a regular noun in the sentence. This same practice is often used with personal pronouns referencing God as well ("You", "He"). This practice is not a hard and fast rule of grammar orthography, though it is a common practice, particularly among Christians.