Depends which ones. The "us and only us" was a bit of a fantasy (Christianity notably coopting a lot of traditions and holidays) but is mainly an Abrahamic thing. Go east from Europe and the middle East, assuming you start counting at the invention of Islam anyway, and all sorts of religions blend together and take aspects from different ones.
Also more people = more money and power. Has been true since always. If you don't get people to join then you lose out on power and money.