According to the game's art director, Tatsuya Yoshikawa, The Order of the Sword is based on the Amish religious sect as a means of blending the medieval elements within the modern setting.

"We brought up the Amish early on, although the goal was never to focus on emulating them too much. I think any given religion represents a closed-off space, in a way. The Order tends to dress its knights in these flashy costumes, but there's a slightly modern sense to them. If they were really shut off from the world, there wouldn't be any modern elements in their costumes at all, right? Those designs were meant to show that the Order has adapted to modern times while stilt retaining their gaudiness. That was our goal there."
Contributed by DrakeVagabond