|From email conversations, slightly edited for clarity (my contributions only).|
As I understand it, the sociology of religion attempts to be a neutral objective study without interest, whereas religious sociology was done by churches to further their interests, particularly French Catholicism. I've never found this distinction convincing. For some, sociology of religion has always been a conservative area of sociology. It also seems inescapable regarding bias, either for religion or not treating it seriously (ie from the perspective of believers) or uneasily wrapped up in Christian theology. Well, seeing as it can be argued that secularisation has a lot of the post Christian about it, there is always going to be some overlap between sociology of religion and theological elements, even if unapologetic, but it is a relationship that should be treated with care. The best approach is to declare a religious position and get that out of the way, so that the sociology can be judged on its merits and its bias.