What is the difference Foster makes between “religious knowledge” and “religious belief”, and why is it important to recognize and maintain that separation while reading and analyzing texts?

From the text:

".....if you want to read like a literature professor, you need to put aside your belief system, at least for the period during which you read, so you can see what the writer is trying to say. As you’re reading that story or poem, religious knowledge is helpful, although religious belief, if too tightly held, can be a problem. We want to be able to identify features in stories and see how they are being used; in other words, we want to be analytical."