Which Roman Emperor forbade pagan cults and festivals, including the Olympics, in 339 A.D.?

The Roman Emperor who forbade pagan cults and festivals, including the Olympics, in 339 A.D. was Theodosius I. His reign marked a significant shift in the religious landscape of the Roman Empire as he actively promoted Christianity and sought to suppress pagan practices. Theodosius issued the Edict of Thessalonica in 380 A.D., which declared Nicene Christianity as the state religion, leading to the decline of pagan rituals and public ceremonies, including the Olympic games that were linked to of pagan traditions. His opposition to paganism was part of a broader effort to unify the empire under a single Christian identity, emphasizing the shift from polytheistic traditions to a predominantly Christian society during this period.

Constantine I, while he did support Christianity and established Constantinople, did not specifically outlaw the Olympics or all pagan festivals during his reign. Nero is often associated with persecution of Christians and other activities but didn't focus on the prohibition of paganism in the same manner. Julius Caesar predates this time significantly and was more associated with the expansion of imperial worship rather than the suppression of pagan traditions. Theodosius I stands out for his decisive actions against paganism, making him the correct answer.