[MMM]

It isn't something I want to argue about, but the definition of the cultural activities visa seems clearly NOT religious:

academic or artistic activities that provide no income, activities for the purpose of pursuing specific studies on Japanese culture or arts, or activities for the purpose of learning and acquiring skills in Japanese culture or arts under the guidance of experts (for example, ikebana, tea ceremony, judo, etc.).

Becoming a monk is neither an academic or artistic activity. Look at the examples they give: ikebana, tea ceremony, judo...none are religious in any way.