Sumo originated in Shinto religious rituals whereby a human would wrestle with a kami a Shinto divine spirit. There are still sumo rings on the grounds of many shrines in Japan.
Shinto ritual guides every aspect of sumo. Shinto referees gyoji essentially act as priests in the ring. For example they perform purification rituals to rid the ring of evil spirits before a match.
The wrestlers themselves throw salt into the ring before each match to purity it.