Ambrose of Milan

Ambrose was a Roman governor who got drafted to be bishop while still a catechumen — he wasn't even baptised yet. He used the office to make Christianity fit into the imperial Latin world: he composed hymns, wrote on the sacraments in classical Latin, and faced down the emperor Theodosius until Theodosius did public penance. He also baptised Augustine. Without him there's no Augustine, no medieval Latin liturgy, and no idea that bishops can hold emperors to account.