The sanctuary of Delphi became well known in the 6th century BC for the Oracle of Apollo, and was regarded as the center of the world (Omphalos). The Pythian Games were held in Delphi every four years, and were comparable to the Olympic games held in Olympia. Delphi started to decline after it was conquered by the Romans in 191 BC and with the spreading of Christianity. Theodosios the Great, the last emperor of the undivided Roman Empire, abolished in 394 the cult of Apollo and the celebration of the Pythian games.