Hey kiddo! So, you know how sometimes people want to protect things or fight for things they believe in? Well, a really long time ago, some people believed that they were fighting for God and protecting His land. They called themselves the "militia dei" which means "God's army".
Basically, these people thought they were fighting a holy war to protect God and His people. They were made up of knights and soldiers who went on big adventures and battles to protect what they believed in.
Nowadays, people don't really use that term anymore because it's not really seen as appropriate to use religion as a reason to fight. But back then, it was a big deal and people were really passionate about it.