Islam is the fastest growing religion in the World. Meanwhile, the West, traditionally identified as Christian, is dying out, mainly because we aren't reproducing ourselves.
It is, but the militant spirit in it is giving way to soccer Islam. Perhaps that is why it is growing.
You comment is not a negation of his. The truth is, the old militant Arab civilization is fighting a losing battle for survival, and the main "attack", although the attackers themselves don't even know they're attacking anything(or perhaps they're attacking Christianity and the effect splashes onto Islam), is in the culture. Western culture has such influence that other cultures, such as Islam, are being flooded with new elements they cannot absorb without losing the things that were most important to their traditional self-definitions.
But the West itself contains some tendencies that might, if left unchanged, bring about a Rome-style decline(the early stages have probably begun, but we are by no means close to the point of no return). If Islam can survive its immediate crises, it just might win. Or maybe not.