The principal challenge to Christianity in the 20th century came from militant atheism in the form of Marxist theology and communism.
It's becoming clearer by the day that the principal challenge of the 21st century will come from Islam.
I've constantly heard Hugh Hewitt disagree with callers who assert that Islam is an all-conquering religion. Maybe Hugh will be more open to the idea after this interview (and Hugh has the most interesting guests in Talk Radio, IMO).