As Mary noted, the main genealogical benefit of religious categories is that people tended to participate in religious groups with family members and the groups kept records. If the person's own religious pilgrimage seems worth writing about, it would seem like the biography is the best part for that. That would certainly seem the case with Charles Darwin. I think he may have been baptized in St. Chad's Church, Shrewsbury, if I recollect correctly. The greater genealogical significance is in terms of what organizations he may have left tracks in, rather than what his personal faith, or lack of faith, was! There is no reason, too, that he could not have more than one categorization, so St. Chads if his baptismal record was there, and if he affiliated with an agnostic or athiest society at some point, then certainly a category for that entity as well.