How closely is the Faith Militant (and in particular the Warrior’s Sons) based on the historic Christian knightly orders (Templars, Hospitallers, Teutonic Order, Knights of Santiago, and so on)?

The Faith Militant is a pretty close Expy for the monastic orders, with some differences. For one thing, the Poor Fellows are more reminiscent of the Peasant’s Crusade, with a strong dash of the peasant revolts of the 14th century.

The Warrior’s Sons are a lot closer to the literary imagination’s version of the monastic knightly orders – they’re knights, they protect pilgrims and holy sites, etc. The main difference, I would argue, is that (compared to the Warrior’s Sons who seem to answer directly to the High Sparrow) the historical orders were a bit more loosely affiliated with the church and more responsive to their own elected leadership, although there are some substantial exceptions where popes got more directly involved in their activity.