As far as I can assertain Christainity is the largest religion in the world today, just ahead of Buddhism. The vast majority of religions believe in one God. It's therefore easy to depict that "one God" as the same. So, the major religious beliefs all pray to the same God. Yet only one believes salvation comes from his son.
Now, just for fun, let's say you are Satan. And you're really pissethed that your enemy, (God), has all these followers worshiping Him, and you want to devise a plot to confuse and disrupt his superiority. What would you do?