[This is the beginning of reposts from my replies to questions posted on Quora, a question-and-answer site where questions are asked, answered, edited and organized by its community of users, at quora.com. The questions and their subtexts are not mine.]
Q: What's a good estimate of believing Christians?
The official number is around 2 billions but for example in my family we are all (6) baptized but only 50% are real Christians, the other 50% are atheist but still nominal Christians.
A: My rule of thumb, about any religion or none, is about 10% really know, understand and assent to the fullness of their nominal faith. Of those, probably none live out such beliefs perfectly. Augustine of Hippo, bishop, scholar and saint, wrestled with this question when he compared the visible ecclesiastical community with the communion of saints, dead and alive, known only to God.
As for Christian statistics in particular, there are many problems. First, different denominations count members in different ways. Many Protestant denominations count only confirmed members. Catholics and Orthodox count baptized people. That already distorts the number.
Sociologists of religion tend to prefer, as a yardstick, behavior indicating adherence. Here again, there is the problem of finding measurable behavior. A common, if imperfect yardstick, is attendance or participation in ritual services or events beyond major holidays. People who go to church one a month throughout the year, for example, are likely to be more seriously committed to a belief (regardless of whether they succeed at meeting the faith’ s ethical demands) than people who go just for Christmas and Easter.