Complex sentence: They formed an order which continued to operate until, during the occult revival of the mid- to late-nineteenth century, the order grew to its maximum of 22 members and split into separate German and British orders.
Split into simpler sentences: They formed an order which continued to operate until the 19th century. During the occult revival of the mid- to late-nineteenth century the order grew to its maximum of 22 members and split into separate German and British orders.