I've been wondering about how to entice other countries to join my (Protestant) religious league. So what factors make nations more likely to join one side or the other? If numerical weightings could be provided, that would be even better.
For princes in the empire, they decide based on their religion. For non-HRE nations, they join the side that opposes their rival.
This usually leads to a chain of large nations joining: Austria is usually the Emperor, so Austria's rivals joins the Protestant league. Then the rivals of those rivals join the Catholic league and so on.