Criteria of religion contested
There are no generally accepted criteria on determining what is a "religion", and that's where the problems start. One could limit it to self perception, but this does not always work. For example: the Free and Accepted Masons of the Johannes Lodge, having three grades, apprentice, brother member and master, do not regard themselves as a religious community, or, to be more precise, not all Freemasons do. However, in the higher grades the ceremonial acts have all the earmarks of religious rites to such a degree that high standing free masons have fewer problems being classified as a religious group. Viewed from the outside, it all appears different. In each of the three grades the religious elements are so dominating, i.e. with regard to references made to a holy text (usually the Bible) or the treatment of death and dying in the grade of master, so that many religious scientists classify Freemasons as a religious community. But that is the extramural view.
We have tried to take these considerations into account by listing Freemasons in a column citing religious and philosophical groups. This concerns many communities: does the Transcendental Meditation have religious or philosophic character – the latter is assumed in Germany. Are Scientologists guided by religious practices or rather, as critics insist, by economic interests? Is Anthroposophy a philosophic movement, as the adherents suggest, or is it a religious community in light of their aim to recognize the divine and exclusion via reincarnation? We have made case-by-case decisions, instead of general assessments.
That raises the issue of random and arbitrary definition. We are aware of the issue and there is room for only one fitting response: we will disclose our decision guidelines and open them for discussion.
Problem of membership count
An additional problem concerns the membership count. As long as there are registered members, the number of members may be grounded on this figure, i.e. the major churches. However, how should members be counted that never attend ? Or the (smaller) group of people that work in a large church community, but are not formal members themselves? With Muslims, who do not have a formalized membership, everything becomes even further complicated. At most, the members of a mosque group may be considered formal members, unlike the participants to the Friday prayer rituals. One could count the worshippers attending the prayer rites, but only those members will attend that need not go to work. A larger group would be the holiday worshippers, but usually the women are missing. Their census offers additional problems, because women usually meet in separate meeting halls and are oftentimes not included in the visitor's numbers; children were not even considered yet. What about esoteric movements that have loose commitments and a high rate of fluctuation? How should one deal with "hybrids" – people who combine a number of religious traditions and feel kin to different groups – even holding double memberships ? The following applies in all these cases: there is no general criteria. Decision guidelines must be made and openly disclosed.
With these dimensions in mind, the maps on religious diversity are a depiction weighed by many preliminary decisions. Nonetheless, the basis for such maps may be calculated and displayed with a reasonable lack of definition.