Christianity was not the only religion that was gaining popularity within the Roman Empire. Various forms of sun worship were also attracting adherents, among whom were the emperors themselves.
Mithraism was especially popular in the military, because it was exclusively for men and promoted brotherhood and fellowship. It had a hierarchical structure where men could advance from grade to grade, just like in an army.
Other forms of sun worship were also in vogue. When Nero commissioned a gigantic statue in his own honor, it featured a likeness of the emperor's head in sun-god fashion. Known as the Colossus of Nero, it stood 37 meters high. Future emperors would alter the features and dedicate it to the "unconquerable sun."
Aurelian, emperor from 270-275 AD, established a state religion that included worship of the emperor and the sun, Sol Invictus. Diocletian, whose reign began in 284 AD, was also a devotee of the sun god.