Ha! Got 400 million from the web. I have seen estimates as high as 500 million. Either of these figures would have to include all of the charismatic subfamilies. The two new links on the page are excellent. Hadden's 9 million is US, and is definitely low. He only counts "established" churches. There are *lots* of independent congregations in this tradition. 400 million is probably high, even worldwide.
Pentecostalism is less a denomination than a family of beliefs and worship traditions. In a nutshell, add Acts 6 to the Nicene Creed and you have the basis of doctrine, add electric guitars, a drum set and (optionally) a brass section to the choir, presto, worship (Yes, can be very loud, as in "raise the roof" loud. Someone explained it to me once: "We don't believe Satan should have all the good music.") Pentecostalism might be a better name, maybe even Modern Pentecostalism. Somebody else can refactor, rename whatever, or I will do it later.
The spread is world-wide, but it appears to be growing fastest in the Americas.
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