This was in part a response to advice Diocletian received at the oracle of Apollo at Didyma.Previous to this, he had purportedly only advocated banning Christians from such things as the military and ruling body in hopes that would appease the gods.
In 39, there was a conspiracy against him in the Rhine area legions.
Upon Tiberius' death in 37, the Praetorian Prefect Macro, an acquaintance and ally of Gaius, proclaimed the latter as Princeps, and the Senate ratified the choice.
Gaius was better known as Caligula, meaning 'little boots'.
Prior to the 6th century AD, many Christians who didn’t use an Anno Mundi (in the year of the world) type system relied on Roman dating, either marking dates from the year legend had it that Romulus and Remus founded Rome (753 BC) or by relying on the date system established under the Roman emperor Diocletian (244-311), based on the accession of Diocletian.
However, most Christians weren’t too fond of Diocletian, since he brutally persecuted them in the latter part of his reign in the late third / early fourth century.