one of the main competitors to Christianity in Rome was Mithras, whose offshoots were all over the area in the few hundred years before & after Christ...Mithras was very popular in the army.

Lots of factors led to Christianity getting a foothold, but if the Emperor Constantine had not had a 'vision', and decreed that worship be switched from the 'old' gods, it is likely that the Christians would be just a historical blip. After a few decades of 'visibility', they were pretty well established, and had a good story and system which was more adaptable and had more interesting 'promises' than Mithras or Jupiter.....folks do like the idea of eternal life, and Mithras didn't allow women to join, if I remember right.