One of the questions that bedeviled me when I first became an atheist was why Christianity went from being a movement within Judaism only to expand outwards to embrace non-Jews.
I had always taken it for granted that Judaism was an insular religion in that Jews believed that they were the chosen people of the Biblical god and that it was not their concern what gods other people worshipped so long as the Jews were left alone.
What I did not realize for many years afterwards was that there was a time when Jews did actively proselytize and seek to convert non-Jews to Judaism. For example, Herod the Great was an Idumean (Edomite) whose ancestors were forcibly converted to Judaism by the Hasmoneans (Maccabees). I was surprised to read that Jews comprised some 10 to 12% of the population of the Roman Empire in the first century C.E., and that such a huge number could only have come about from Judaism aquiring converts from people of other faiths.
Furthermore, as Rodney Stark writes in his book "Cities of God", early Christian preachers such as Paul did not primarily convert Gentiles to Christianity, but rather fellow Jews to Christianity. Stark argues that it was easier for Jews to convert to Christianity than pagans because of something he calls "religious capital." A Jewish convert to Christianity would still be worshipping the same god and his holy texts were integrated into the new faith. Thus, with the Jewish Diaspora spread out across the Roman Empire, it is easy to understand how Christianity took hold so early in various parts of the Empire. There was already an available supply of Jews to tap into.
However, this still does not explain the "why" of Christianity. Why did it happen? (I know, if you are a Christian, the answer is that you believe that Jesus Christ was born from a virgin, was crucified and rose from the dead, and commanded his disciples to spread his message.)
One way of explaining how it came about by way of analogy occurred to me when I learned about the term zoonosis. To put it simply, zoonosis is when a pathogen that makes the leap from animals to humans. Examples of zoonotic pathogens are the HIV virus, the Ebola virus, Lyme disease, and West Nile Virus. Take Ebola for example. It has been determined that its reservoir host is three species of fruit bats. But every once in a while, Ebola comes into contact with humans.
It is widely believed that humans are at greater exposure to zoonotic pathogens because of human encroachment on animal habitats, which bring the reservoir hosts into greater contact with humans.
So where does Christianity fit into this? First off, so as not to offend Christians, I do not mean to imply that Christianity or Christian believers are a virus or disease. I am merely using zoonosis as a model to describe how Christianity grew out of Judaism and spread to become the state religion of the Roman Empire.
Just as human encroachment on the habitats of reservoir host animals facilitates the transfer of pathogens from the hosts to humans, so Roman expansion into the Middle East served as a catalyst to spark the creation of Christianity. The Jews were the reservoir host for Judaism, of course. But in response to the encroachment of Rome on Judea, some person or group of persons was inspired to make a mental leap and jettison the onerous rules of Judaism and refashion it into a universal religion that applied to all people, whether they be Jew or Gentile.
I have lately come to believe that Christianity would not be possible if it were not for the Roman Empire. The Roman Empire was an ideal host for the new religion. Rome ruled over a vast territory that stretched from the Middle East to the British Isles. Thus, Christians had the means to spread their faith over a large geographic area controlled by a single political entity, instead of encountering a patchwork of states with fortified borders and generally hostile relations.
True, there were occasions when the Romans persecuted Christians, sometimes quite vigorously. But just as the size of the Roman Empire helped to facilitate the spread of Christianity, it also helped to hamper the ability of hostile emperors to persecute the faith. It would often take months for imperial edicts to be transmitted to the most distant provinces of the empire. Even then, implementation of the policies was rarely enforced with any degree of uniformity. A governor of a province with a small Christian population would likely be much more vigorous in carrying out anti-Christian policies than a governor of a province that had a large Christian population simply because it was easier to do so. A governor ruling a province with a large Christian population might balk at the prospect of encountering resistance from his Christian subjects, or he might even have friends or family members who were Christian converts and thus be sympathetic to them. With the passage of several years, a new emperor would sit on the throne and the persecution policy would be relaxed or repealed.
As I mentioned above, the Ebola virus is an example of a pathogen that is transmitted on occasion from its animal host to humans. As odd as it may sound, it is actually a blessing that Ebola kills its victims so quickly. Humans who have contracted Ebola thus far lived in rural areas and they tend to die before they come into contact with large population centers. Other pathogens, such as HIV, do not kill their hosts or even indicate their presence for many years, thus facilitating their spread from one person to another. As a zoonotic pathogen, Christianity more resembles HIV than Ebola.
Christianity was very fortunate in that its host, the Roman Empire, was an enduring state, which gave the new religion the time it needed to spread until it reached the tipping point and became the de facto religion of the Empire under Constantine and the official religion under Theodosius in 395 C.E. Had the Roman Empire split apart into a multitude of mini-Romes during the civil wars of the 3rd century, some of the off-shoots of the Empire might have been Christian states while others remained pagan. On the other hand, if Rome did not extend itself into the Middle East during the 1st century B.C.E. onward, I suspect that Christianity might never have happened at all.