Growth/attrition rates are actually the thing to look at here. Scientology is faster-growing than just about any other modern religion, though the attrition rate is also very high. In order to figure out virulency, figure out what population the S-curve of members of that religion will top out at. If growth is slowing, you're almost there. If growth is steady, you're about halfway there. If growth is exponential or approximately so, you're looking at a religion in its infancy.