Nature Abhors a vacuum

"The effects of European secularism, you see, are not limited to the children of Muslim immigrants. A culture where barely half the people believe in God, and far fewer practice any religion at all, cannot compete with Islam's vitality. While Europeans have ceased believing in God, they and their children have not stopped needing Him: Their need for meaning and purpose has not gone away. They have just been convinced that these will not be found inside a Christian church. And they certainly will not find them in the secular welfare state. So for many that leaves only one alternative: radical Islam... It's a choice that makes sense only when you realize that we are by design meaning-seeking creatures."

—Chuck Colson

(Thanks to the Federalist)