Perhaps a good rule of thumb: any time a significant number of worldwide religions independently converge on a single practice/idea, there may be something of value there worth porting to the secular world (see also: the sabbath, and the "golden rule").
If I ever retire and become a cult leader this is my pet religion, I call it Vennism. Combine all the teachings and trim away those teachings that aren't common, what you're left with seems like it would probably be a good religion right?
Sounds good, but no kool-aid. We gotta mix all the other religious drinks instead. Thinking like a trippy sangria. You got the Christian wine, some Shamanic ayahuasca, Buddhist tea, Muslim qahwa, maybe a little blood of the innocent for the Satanists out there.
