In Frankenstein, what might Shelley be saying about religion? In particular, what might she be saying about Christianity?
Mary Shelley's Frankenstein grapples with a variety of themes, including religion, and especially Christianity. While many things could be said about the novel's dealing with Christianity, I'd like to focus on one reading in particular: it's possible to see Frankenstein as Shelley's exploration of the possibility of a God who not only does not care for His creation, but who also detests it.
Consider the religious parallels here: Victor uses advanced knowledge to create life, an act that could be seen as analogous to the creation story in Genesis. Additionally, the Monster could be seen as a parallel to the first human created by God. Indeed, the Monster actually compares himself to Adam. Like Adam, the Monster is the first of his race and, also like Adam, he must suffer exile in the wilderness (although Adam has a companion at that point, while the creature does not). As such, it's possible to see the whole novel as at least partly analogous to the Christian creation story. There are of course other things going on here, but one can't deny that the religious connection is considerable.
In Shelley's universe, however, there is one major difference: unlike Adam, the Monster does not expressly rebel against his creator, and yet he is still cast out into exile. The Monster goes on to murder and commit other horrific acts, but he ultimately does so because he is unjustly rejected by his creator, Victor Frankenstein. As such, Shelley's parallels to Christianity become very disturbing indeed, as she seems to be exploring the possibility that God is not a loving deity, but is rather disgusted with His creation and has rejected it. In that case, Shelley's depiction of Christianity takes on a critical tone, as she seems to be calling into question, and even critiquing, the perception that God is a loving creator.