In this example, "God" is a proper noun. This being's name (in the religion of the person who wrote this book and for most Christians*) is God.
The word of Peter, the word of Mom, the word of God.
The word "god" can be a countable, generic noun as well. When we refer to the Roman or Greek gods, for example, we don't call any of them "God" -- we call them by their names, such as Zeus or Hestia.
* Please do not turn this into a theology thread by posting all the names by which the various religions call God. It won't change the grammar. If you want to discuss this, post it in the discussion forums, please.
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