There are other ancient traditions besides Christianity. It might be worth looking at their equivalents or near-equivalents, to see what's common between them (or particularly insightful in just one).

You probably want to choose traditions that have lived for a long time, not merely existed for a long time. If someone writes a book and it survives 2000 years, that's some evidence that it's got valuable insights in it; but if someone writes a book, and then other people work on refining its ideas for 1000 years, and then the results survive another 1000 years, that's probably better.