This story has received a lot of play, but isn't this comparing apples to oranges? There are still more Christians than there are Muslims, and Muslims, like Christians, are not just one group. Most obviously, there are Sunni Muslims and there are Shiite Muslims, and if I understand things correctly, there are some divisions within those major groupings as well. I'm not sure why it matters, but until (at least) there are more Sunni Muslims than there are Catholics, I think Catholicism is still the world's biggest religion by adherents.