The definition of "religious affiliation" or "religious denomination" is central for any statistic on this aspect of social structure. For the purpose of the SMRE, religious affiliation is defined as an institutional relationship between a person and a religious group or tradition.
In many European countries these institutional affiliations are rooted not only in historical traditions, but moreover in an official membership status which in some case even establishes the right to tax the members of certain churches or religious communities. Traditionally in Europe this kind of religious affiliation is an exclusive social relationship. Resulting from the «cuius regio, eius religio» approach so prominent in the religious wars across Europe in the 16th and 17th centuries, in Europe being a member of one church usually automatically means the exclusion of all other membership options of organized religion.
Thus, religious affiliation as used in the SMRE is defined as an "objective" property of a person and of a society’s social structure. Religious affiliation is a substantial aspect of religious belonging, but – leaving out the more subjective dimension of individual religious persuasions and emotional identification with a religious group or tradition by definition – both concepts are not identical. In practice, the list of categories useful to aggregate and integrate data of different sources into the SMRE consists of:
- other Christians
- no religious affiliation