[Michael Goldsberry]

Seems to me the Muslims have just pissed off some 1 billion Catholics.

That figure is roughly correct.

Source

Elsewhere...

[Junior]

Can you imagine the retribution should some follower of the Prophet (cursed be his name) should off the Pope? Your average European isn't all that religious, but he or she still takes some things seriously, especially in southern Europe. There wouldn't be a Moslem alive south of the Danube within a couple of years.

[Tench_Coxe]

Interesting breakdown:

Major Denominational Families of Christianity
(This table does not include all Christians. These numbers are estimates, and are here primarily to assist in ranking branches by size, not to provide a definitive count of membership.)
Branch Number of Adherents
Catholic 1,050,000,000
Orthodox/Eastern Christian 240,000,000
African indigenous sects (AICs) 110,000,000
Pentecostal 105,000,000
Reformed/Presbyterian/Congregational/United 75,000,000
Anglican 73,000,000
Baptist 70,000,000
Methodist 70,000,000
Lutheran 64,000,000
Jehovah's Witnesses 14,800,000
Adventist 12,000,000
Latter Day Saints 12,500,000
Apostolic/New Apostolic 10,000,000
Stone-Campbell ("Restoration Movement") 5,400,000
New Thought (Unity, Christian Science, etc.) 1,500,000
Brethren (incl. Plymouth) 1,500,000
Mennonite 1,250,000
Friends (Quakers) 300,000

Major Branches of Islam
Branch Number of Adherents
Sunni 940,000,000
Shiite 120,000,000
Ahmadiyya 10,000,000 *
Druze 450,000

from here:Religion Demographics