Re: Catholicism
I'd assume it has to do about the level of civilization, and our understanding of the universe. After all, look at other religions that 'arose naturally' over time. When the first men were alive, you mostly had shamanism, individuals who didn't report to anyone, and didn't work towards social unity. If Jesus came to one tribe and said 'follow me!' it could have spread to a few neighboring tribes, but it would have changed significantly, and wouldn't have gotten far.
You have the first organized religions with Sumeria, but even then, had Jesus come, a few traditions would have been carried off, but you would see only a fraction of what Christianity is now, if that.
The Romans were the first 'great empire'. They spanned from the Atlantic to the Pacific (almost). Their only real competition was China and India. The Romans covered three continets, had inherited both the Greeks and the Sumerians. They had an empire-wide organized religion. Yet, they didn't have anything that applied to the larger, poorer classes like Christianity would.
As you saw, Christianity spread slowly at first, but became very popular with the poorer classes. It then became the official religion, and the entire empire of Rome spread it half way across the globe.