That's an erroneous claim. The Muslims are all divided among themselves just like the Protestants, Eastern Orthodox, and ALL religions except one. The Catholic Church is one. If 800 million Catholics died tomorrow, it would still be the worlds largest religion.
Religion / Number of Adherents
Zoroastrianism / 110,000
Shinto / 3,000,000
Jainism / 4,000,000
Baha’i Faiths / 6,000,000
New Japanese / 10,700,000
Judaism / 14,900,000
Sikhism / 23,000,000
Buddhism / 329,000,000
Protestant / 667,100,000
Hinduism / 832,000,000
Catholic / 1,050,000,000
Islam / 1,071,000,000
How can you say this??? The catholic church itself has numerous branches and they can differ considerably from that which Rome teaches....Take a look at the Haitian Catholics who incorporate Voo-doo into their catholic faith. In fact a Voodoo priest must first agree to become a catholic before he is given the title of a Voo-doo Priest. I doubt most catholics will adhere to their church and yet they are indeed monetary recipients of monies from the catholic church.