Islam has already conquered two-thirds of Christendom. After a pause that's lasted centuries, they are ready to expand further. I predict that they will grasp two-thirds of the remainder of Christendom before the West figures out what's going on and moves to stop (not reverse) their expansion.
You are probably referring to what was Christendom at the time of their earlier expansion -- but "Christendom" has also expanded since then, into the Western Hemisphere, Africa, and Australasia. They may take a good piece of what used to be the Holy Roman Empire if the Euros don't get their head straight pretty soon, though.