Constantine the Great

When Constantine returned from the eastern front to Nicomedia in the spring of 303, he witnessed the beginning of Diocletian's "great persecution," the most severe persecution of Christians in Roman history.

An excerpt from the article 18 facts about Constantine the Great

Diocletian ordered the destruction of the new church in Nicomedia, the destruction of its writings and the confiscation of its goods. In the months that followed, other churches were destroyed and priests imprisoned.
It is unlikely that Constantine played any role in this persecution. In his later writings, he tried to portray himself as an opponent of Diocletian's "bloody edicts" against "God's followers." However, there is no indication that he successfully opposed them.