Emperor Julian II is better known under the name of Julian the Apostate (360-363). Indeed, Julian was the only Roman emperor to convert from Christianity to paganism, and is considered to have been the last pagan Roman emperor.
Soon after his ascension to the throne, Julian started to publically promote the ancient cults, and to personally participate in the ceremonial of sacrifices. On his coins, the emperor had himself depicted with a beard, following the tradition of neoplatonic philosophy. The bull on the reverse of this bronze coin stands for the sacrifices that were so important in the original Roman religion.