This is a great question to consider.
I've been thinking about it alot.
Here in this clip Tim Keller (Lead Pastor of Redeemer Presbyterian) gives a really good description of why Christians can easily end up introverted and insular to the people God has placed around them.
Groups of Christians do this when they separate themselves from and actively oppress not only nonChristians but also OTHER Christians who aren't "just like us."
Step One - Superiority. "I/We have the truth and they don't."
Step Two - Separation. "I no longer spend time with them, they aren't up to the mark or even impure.
Step Three - Caricature. "Because I don't actually spend time with them to know them, I caricature them. Result, they now become one dimensional."
Step Four - Passive & Active Oppression. "Now Caricatured they become the type of people I can push away and ignore. Passively at first, then over time, actively."
Check out Keller's new book "The Reason For God" here!