Constantine the Great

As a future candidate for emperor (there was an informal succession of privileges in the Tetrarchy), Constantine stayed at the court of Diocletian, where he received a proper education.

An excerpt from the article 18 facts about Constantine the Great

He studied Latin literature, philosophy, and Greek. He spent time in the culturally diverse environment of Nicomedia (the eastern, oldest capital of the Roman Empire, now a city in Turkey), where he met both pagan and Christian intellectuals. He may have attended the lectures of Lactantius, a Christian Latin scholar. Diocletian did not fully trust Constantine, who was treated as a hostage at his court. This made Constantine obedient to Diocletian.