And of those billion followers, how many live in countries where any other religion is banned or wiped out ? And, how many are children who are not capable of choice ?
Well, take Indonesia with about 200 million. It's a weird mix. Technically about 89% Muslim. But there are some weird muslim strains there that have combined with sufism and hindu and local ancient religions. About 7% is Christian and the other 4% a stew of traditional regional religions and Hindu etc.
Egypt has coptic population, which has lately come under attack. Places like Jordan, etc. have populations of eastern orthodox, which is a very early form of christianity.
The only country that I know of that has essentially banned other religions is Saudi Arabia, but they are an American ally.