Cauf approaches it from an entirely secular angle I know, but it got me thinking about the differences in religious doctrine, and how the different groups may not be able to coexist based on that alone.  Honestly, I don't even know what questions to ask here, but I know this is about more than just immigration.

Edit: I figured it out.  How should Christians respond to a religious war against them, not just from the outside, but from the inside?  Suppose, for example, a large group of people move into your community and start saying some un-Christian things in church, with the intent of corrupting the faith?  Do you gatekeep, and if so, how?

Second edit: when I say "church," I don't mean a dedicated religious building, I mean any gathering for spiritual study.  In fact, forget my original question, it was late, my mind has been a bit foggy from lack of sleep the past few days.  This really isn't about arbitrary physical boundaries anyway, this is more about ideas.  One reason to be wary of massive movements of people is the ideas they spread, so how do Christians keep bad ideas in check, both within and without?  I hate to get all mediaeval here, but what is the proper approach to heresy?  By the way, it's not just Christianity that needs an answer; *all three* of the Abrahamic religions have been infiltrated by esoteric cults.