As you know, the most important part of a paper is the thesis statement. Christianity is such a broad topic that you will need to choose a thesis statement that is focused. Let me give you a few examples.
- "Christianity is now a more non-western religion than a western one." This can be a great thesis, as the church is growing in continents like Asia and Africa at an incredible rate. Philip Jenkins has written a lot of this topic.
- "Christianity helped to create a scientific worldview." This is a debatable topic, but all thesis statements are. You can argue that because Christianity believed in a God who does not change this enabled people to study the uniformity in nature, which helped to bring about a scientific worldview. You might want to read some Rodney Starks on this topic.
The key point to keep in mind that you need to narrow things down in terms of a thesis and support your thesis will good points.