What/who was responsible for Julian’s paganism?

If Christianity was the new ‘trend’ of religion, and Julian’s upbringing was closely monitored and isolated from most people, how did Julian pick up paganism? I read that he was placed under the care of Eusebius, who was a Christian and (as he claims) very close to Constantine. With this sort of influence on Julian’s young mind, how did he end up being a dedicated pagan?