Dogen gave Soto its practice. Keizan, four generations later, gave it the institutional form that made it the largest Buddhist school in Japan, and the two are often in tension.
Expansion by accommodation
Dogen wrote for a small community of full-time practitioners and was severe about mixing the practice with anything else. Keizan and his successors took the school into the provinces by doing the opposite: taking over existing temples, absorbing local kami cults, performing healing and rainmaking, and offering esoteric rites to lay patrons. Soto grew from a handful of temples to thousands within two centuries, and it grew through what Dogen had warned against.
The two ancestors
The tradition names both men as founders, Dogen as the eminent ancestor and Keizan as the great ancestor, which is an unusually honest way of recording that a school owes its survival to a change of direction. Modern Soto scholarship has argued at length over how much of the accommodation was betrayal and how much was the only available road.
The funeral economy
Under the Tokugawa temple registration system every household was attached to a temple, which kept its records and buried its dead. Soto had the widest rural network and gained the most. The consequence is still visible: most Japanese contact with the school is at a funeral, the phrase funeral Buddhism is used inside the tradition as a criticism of itself, and the temple-household relationship that guaranteed income for three centuries is now dissolving as the countryside empties.