I just had a realization based on your comment; Their boots are dirty because they are walking through a world of shit. That is a core tenant of Christianity. They get to cop out on trying to make anything better by just saying “oh well, the world is shitty, get believers to make god daddy happy and then die.” That lines up with Trump’s thematic message of America being some over run, crime infested wasteland. It is the hook by which Trump catches the Christians.