CONSTANTINE (272/273–337), known as Constantine the Great, Roman emperor and agent of the Christianization of the Roman Empire. Born at Naissus, the only son of Helena and Flavius Constantius, Constantine was assured a prominent role in Roman politics when Diocletian, the senior emperor in the Tetrarchy, appointed his father Caesar in 293. Educated in the imperial court at Nicomedia, and permitted to accompany the eastern emperors on provincial tours and military campaigns, he doubtless expected to succeed to his father's position when Diocletian and Maximian abdicated in 305. But Galerius, who may have contrived the abdication and as the new eastern emperor controlled the succession, ignored Constantine—and Maxentius, the son of Maximian—and instead nominated as Caesars his own nephew and the praetorian prefect Severus. Constantine could not challenge this decision immediately, but when his father died at York in July 306, he reasserted the claim, this time backed by the...