The Bible isn't exactly a symbol as much as a pattern of imagery, but once you start looking it's all over. Check it out:
But don't take our word for it. Here's the monster, talking about reading Paradise Lost:
Like Adam, I was apparently united by no link to any other being in existence; but his state was far different from mine in every other respect. He had come forth from the hands of God a perfect creature, happy and prosperous, guarded by the especial care of his Creator; he was allowed to converse with and acquire knowledge from beings of a superior nature, but I was wretched, helpless, and alone. Many times I considered Satan as the fitter emblem of my condition, for often, like him, when I viewed the bliss of my protectors, the bitter gall of envy rose within me. (15.7)
What's the point of confusing the business out of us? These complex Christian allusions steer us away from thinking that we can wrap up our analysis in a neat little package. And what's cool about that is that the form (the complicated text, which doesn't let us neatly analyze anything) matches up with the content (Shelley's little lecture about how we shouldn't put people/monsters into boxes). Pretty nifty.