There is one religion that I think that could challenge Christianity in a world without Constantine, especially if the Roman Empire breaks apart sooner than in RL.
Manicheanism.
Even when Christianity was on the ascent, Manichaenism spread very quickly from Persia in East and Western directions, there was a Manichaean monastery in Rome at one point and of the Gnostic faiths, it was easily the largest that existed. And before people say, oh Gnosticism wasn't THAT popular, the Gnostic religions persisted for over a thousand years and a religion doesn't last that long without being popular in some form.
In RL the religion of light supposedly died out by the 17th century, though there have recently been mentions of some Manichaeans continuing to exist and practicing the faith in secret in China.
Regardless, I could see Christianity splintering into several competing sects, especially if the Roman Empire breaks down faster than it did in RL with no one sect being able to absorb/wipe out the others, and then facing pressures from new religions rising from the east.
Manicheanism.
Even when Christianity was on the ascent, Manichaenism spread very quickly from Persia in East and Western directions, there was a Manichaean monastery in Rome at one point and of the Gnostic faiths, it was easily the largest that existed. And before people say, oh Gnosticism wasn't THAT popular, the Gnostic religions persisted for over a thousand years and a religion doesn't last that long without being popular in some form.
In RL the religion of light supposedly died out by the 17th century, though there have recently been mentions of some Manichaeans continuing to exist and practicing the faith in secret in China.
Regardless, I could see Christianity splintering into several competing sects, especially if the Roman Empire breaks down faster than it did in RL with no one sect being able to absorb/wipe out the others, and then facing pressures from new religions rising from the east.