The “religion2” variable in the historical Canadian censuses has a hierarchical structure in which religions are classified into families. For example, religions coded 1000 to 1999 appear to be classed as part of the “Western Roman Tradition.” What is the source of this classification?
These major categories and sub-categories come directly from the source variables. For example, the CA1871A_RELIGION variable includes the same sort of hierarchical coding structure. The harmonized variable RELIGION2 aims to try to preserve as much of this original detail as possible.
Looking a bit deeper into this, it looks like the Canadian Families Project produced these historical samples. In their User’s Guide they document RELIGION2 (on page 25) groupings as, “adapted from J. Gordon Melton, The Encyclopedia of American Religions (Wilmington, N.C., 1978), volumes 1 and 2”