I believe that there are three separate factors in the growth of a new religion. One, it must be attractive enough in itself that people want to convert to it. Two, it helps if there are real disadvantages to NOT converting to it. And three, strong barriers to "un-converting" will prevent the new religion from losing those converts it has gained.
A good example of all three factors can be found in early Islam. Its message was simple and powerful, so there was a positive attraction to it. In areas that had fallen under its military dominance, non-Muslims were legally disadvantaged at best, making conversion to Islam even more attractive. And renouncing Islam is a capital offense, so once a Muslim, always a Muslim. Under these conditions, it is no wonder that the number of Islamic adherents quickly rose, to where it became one of the world's great religions.
A good example of all three factors can be found in early Islam. Its message was simple and powerful, so there was a positive attraction to it. In areas that had fallen under its military dominance, non-Muslims were legally disadvantaged at best, making conversion to Islam even more attractive. And renouncing Islam is a capital offense, so once a Muslim, always a Muslim. Under these conditions, it is no wonder that the number of Islamic adherents quickly rose, to where it became one of the world's great religions.