One of the more well-known incidents of Emperor Marcus Aurelius’ reign was an event that is called the Miracle of the Rain.
The Legio XII Fulminata and the Emperor were fighting against a tribe called the Quadi, near Carnuntum in Austria. The emperor’s troops got trapped in a waterless valley that was surrounded by Quadi forces.
After five days, the troops were getting desperate. Apparently they resorted en masse to prayer to all their various gods, and the emperor joined in.
Out of nowhere, a rainstorm boiled up. Lightning lashed the mountainsides and thunder drove off the Quadi, while the rain fell so thick that they couldn’t even see the legionaries. Meanwhile, the rain in the valley was refreshing, and the legion was able to fill up their water supplies and then escape the dead-end valley.
(In this sculpture from the Antonine Column in Rome, the Quadi are depicted as getting zotted by a god in a cloud.)
Pretty much everybody in the legion claimed that the miracle was due to their own gods, Dio Cassius said it was a civilian magician from Egypt, and the imperial officials claimed it was because the emperor joined in.
But Christians widely claimed that it was all due to God listening to the Christians in the legion, and indeed there do seem to have been a lot of Christians in it.
Legio XII Fulminata had its headquarters at Melitene, Armenia, and thus is sometimes called “the Melitene Legion.” Armenia was one of first areas converted to Christianity, so it’s not surprising that an Armenian-based legion would have included many Christians. St. Polyeuctes of Melitene is supposed to have been one of its officers, along with his buddy St. Nearchus; and the Forty Martyrs of Sebaste were enlisted men of the legion.
All of these guys were martyred because they were Christians, not because they suddenly refused to fight; they were perfectly willing to fight for the emperor, and for the senate and people of Rome. Also, they were extremely popular early Christian saints. So much for the idea that all early Christians were pacifists.
There may have been another largely Christian legion, if the story of St. Maurice and the Theban Legion is true.