Ethical issues are sometimes addressed in the context of religion; indeed, it’s often assumed that the two are inseparable. For this book, we will reject this assumption, largely for the reasons that Rachels presents, following Socrates. Their reasoning is this:
If some religious text, authority, or even God makes a moral judgment (e.g., about whether some use of animals is morally permissible or not, or any other moral topic), then either there are reasons that justify that judgment or not. If there are no reasons supporting that judgment, then it is arbitrary and should not be accepted. If there are reasons, however, then those reasons are what justify the judgment, not the fact that some authority says so, and we should be able to identify and evaluate those reasons directly.
In sum, “Because I said so!” is not a good reason to believe something, unless whatever is said is supported by reasons. Nevertheless, there are many religiously-motivated animal advocacy organizations and thinkers and the suggested readings and web pages reference them.