Rome was okay with it.

And, we know the Jewish university at Yavne was in on the creation of the Jesus narrative.

Christianity was successful after the first couple generations from Christ because the people who came in that time changed what Yehoshua was actually teaching.

Lets look real quick, there were about 120 in the upper room for Shavuot, those 120 became over 3000 on that day. How? Well they convinced their Jewish fellows that Yehoshua was in fact the Messiah , they did that by demonstrating that he fulfilled prophecies written about what the messiah would do. Those Jews knew the scriptures, and were convinced by these eye witnesses, remember two witnesses prove a matter, 3 establish it as fact, 120 well that is pretty convincing.

So these 3000+ go from Jerusalem back to their homes all over the known world, taking the believe that Messiah has come.

The hard core of these (the 120 and maybe some extras) pool their resources and begin teaching the jews at first. Then there is Paul, who at first spends a lot of time not with the apostles, and with the gentiles teaching them, drum roll please, the Torah. He spends the time laying the torah and prophets as the foundation, and then shows how Yehoshua is the foretold Messiah, eventually he connects with the other believers in Yehoshua.

This means the base of believers grew quickly shortly after the ascension, thats generation 1,

Generation 2, believers like Clement and Origen etc are now the movers and shakers, and from 70 to about 100 ad they have a pretty good field to work with, there is a huge amount of displaced Jews mixed with the nations. At first no one could really tell the difference between a follower of Yehoshua and a believer in judaism, and in that time still more jews are brought into the faith, and many gentiles are brought in too, its not that hard to convince them have faith in something, there is so much faith in many things going on. So it all grows more.

But then when there are many pagans in the faith, and the temple is gone, it begins to go sideways, which actually led to its “success” around 125 ad there was a disagreement in the believers, about when to celebrate Passover, now the temple is gone, and jerusalem is sac’ed so doing the pilgramage is kinda tossed out, but they still were observing passover, not keeping it. But the influx of gentiles began to change things the east and west congregations argued about when to keep the feast, one group said we will stick with the tradition given to us by John, and the other side basically said we will observe the time of celebration according to what the people around us are doing. This is the quartodeciman controversy and it is the key to Christianity’s success.

Once Christianity began to become amorphius the faith began to assimilate other beliefs into itself, and itself began to morph into a different belief than what it began as, and what the gentile believe was that it assimilated.

So of course it spread, anything it touched it adapted itself to mimic, claimed that both beliefs were actually the same, and it spread.

The jews hated them for changing the faith and yet claiming to be of the same faith, so they fought eachother, and frankly not many liked the jews anyways, so they would lean better towards the “christians” this methd of viral growth continued until they had the infiltration of a government, and Constantine saw an opportunity to exploit this ever changing assimilative faith. He could stop all the in fighting by making a Universal faith, a Catholic faith, so thats what happened. And in short order most everyone was a part of this new state sponsored faith.

The Key is the faith changed and would keep changing to tickle the ears of whoever was around it. I mean seriously their sales pitch is, “hey do you want to go to the greatest place not on earth, just believe like us, you dont have to change your life or anything, you just have to admit that Jesus died for you and that he went to heaven. “

So the cost is basically zero and the gain is basically everything? most would say “sure sign me up” plus the government kinda forced people into it.