"How do I worldbuild in relation to Christianity?"
Me: *sees "profile lists as Atheist* Um... okay.
Actually, not being a Christian might put you in the perfect position to base your fantasy works in Christianity. Too many Christian-based stories get too preachy, rather than just developing an interesting world and story.
Here's a few ideas: read some of the Bible. There are a lot of elements of the Bible which are very fantastic. You have prophets who are basically wizards, demons, angels, all manner of crazy creatures (read the book of Revelation for some fun ideas), and magic. I'm working on my own fantasy series where, while not explicitly Christian, does have Christian messages, and magic is a part of that world, because people seem to forget that God has magic, too! I mean, you can call them miracles and try to differentiate them, but Satan was a fallen angel. None of his power, even magic, is something that he made himself. God gave it to him. Ergo, while a lot of people condemn fantasy for "promoting sorcery and witchcraft", I see it as there are two types of magic: good and evil.
A lot of fantasy can stem from Christianity. The golem was something which originally came from Jewish stories of a rabbi making a clay statue and God giving it life. Approaching Christianity as an atheist may be beneficial to you, because you can view it the way people view, say, Greek mythology, and you can build upon it for the sake of creating an interesting world, not for preaching a message.
Want some ideas to inspire you? Read Shadowmancer and its sequels by G.P. Taylor. It's strongly based in Christian belief, but in an interesting and fantastic light that makes it very interesting for anyone regardless of their religious beliefs. Great inspiration if you're going in a somewhat dark, Gothic direction.