There were other cults that got huge too, what became Catholicism (ignoring the east west schism in the 11th century) just won out. For example the Mithraic Mysteries, and several other mystery religions (and outside the empire there was also Manichaeism which was the major competitor to Christianity and Islam in central Asia and Persia, and North Africa for a time, it by far was the biggest competitor, lasting in China till the 14th century. Augustine of Hippo famously converted from Manichaeism to Christianity. It was a syncretic religion, taking aspects of zoroastrianism, christianity, judaism, and Buddhism I think. It revered Zoroaster, the buddha, and Jesus as prophets. Really interesting in that Zoroastrianism also as a base used the same myths as the Rigveda). Christianity was just rather special in the empire in that it was centered more on common practice and salvation than the mystery cults. However it's important to remember that there were many varieties of Christianity super different from one another vying for mass support, and what became catholicism won out via state support and organization like the Nicean councils (as well as outside factors resulting in the decline of others, like the Church of The East). There were versions where Christ was just a man or just a god, where there two different gods, or seven, or hundreds, and all calling themselves "Christianity" (got that from a good askhistorians thread). A quite strange and eclectic one of these, Gnosticism, had ideas get into Manichaeism that made it to China, and still survives in bits in pieces in Mandaeism, a small now ethnic religion in Iraq.