Final answer:
The religion mentioned spread via forced conversion, as the ruler mandated all citizens to adopt the new religion or lose their citizenship, which is a form of hierarchical diffusion.
Step-by-step explanation:
The religion you are referring to in your question spread through forced conversion. This is when the ruler of a country decrees a new religion as the official one and requires all citizens to join or renounce their citizenship. In the context of historical religious diffusion, this would be considered an aspect of hierarchical diffusion, where a religion is imposed from the top down, usually by a central authority such as a monarch or state. This method is distinct from voluntary conversion, missionary work, and cultural diffusion, which represent other ways religions can spread. In some cases, religions spread due to a mix of these factors, including through the efforts of missionaries or traders, the appeal of religious ideas, or through force and coercion.