The real reason why the Christian church was able to thrive actually has a name; Gauis Flavius Valerius Aurelius Constantinus also known as the Roman Emperor Constantine. Constantine is believed to have lived from 272ce -337ce he was the son of Flavius Constantius and Helena. Without getting into too much detail I'm going to get right into what he did for the early Christian church.
Constantine: The true savior of the Christian church!
On July 25 in the year 306ce Constantine was proclaimed an augustus (emperor). As emperor of Rome Constantine is most noted for two pivotal decisions that helped establish the Christian church to eventually become recognized as the church of Rome. The first of these steps was by writing and establishing the Edict of Milan and the second and most influential was the establishment of the first Council of Nicaea.
The Edict of Milan was essentially a proclamation establishing religious toleration for Christianity in the Roman Empire. It was established in 313ce and it gave Roman citizens the right to worship whatever deity they wanted to, gave to Christians legal rights such as the right to establish churches, and ordered the return of all properties that were previously confiscated.
Edict of Milan 313ce
In 325ce the emperor Constantine called together the very first ecumenical council of Nicaea. The main goal of this meeting was to help deal with the heresy of Arianism which was brewing in the eastern church. Arianism basically taught that Christ was not divine but that he was a created being. This heresy was proposed by Arius of Alexandria.
Fist council of Nicaea 325ce
According to some historical sources there were about 300 bishops in attendance and once they had come to an agreement about what was to be done about Arianism they then established the Nicene creed. According to the New World Encyclopedia: "The purpose of a creed is to act as a yardstick of correct belief. A creed is an epitome, not a full definition, of what is required for orthodoxy."
It was said that Constantine was taught Christianity as a youth by his mother Helena but it is uncertain if he accepted Christianity at a young age or if he accepted it later in life. It is known that he was around aged 40 or over when he publicly declared himself a Christian.
Legend has it that on his way to the Battle of the Milvian Bridge 312ce that Constantine looked up to the sky and saw a cross of light above the sun with the Greek words "in this (sign) conquer. It is then stated by Eusebius that Constantine then had a dream where Christ himself came and told him what the significance of his vision had been. The sign is believed to have been a labarum.
Labarum with the Greek letters Alpha and Omega
The New World Encyclopedia confirms two of my theories regarding the union of the church with the Roman empire under Constantine. One of my theories is that once the church had obtained the backing and full force of the Roman empire that they then began to eliminate any competing sects or religious groups. They even went after other Christians sects that they deemed heretics because their beliefs did not conform to those stated in the Nicene creed.
Regarding Constantines conversion: "There has been much speculation as to whether his conversion was genuine or strategic. He certainly hoped the church would be a politically integrating force. And as Christians grew more numerous, it made sense to gain their support."
Regarding the Council of Nicaea: "The council also promulgated a number of enforceable canons to regulate church governance thus creating a disciplined institution. From this time on the relationship between church and state became very complicated with the church co-opting the state to enforce its beliefs and persecute those it deemed heretics."
This was the beginnings of the worlds pain. The persecuted became the persecutor and began paying their former oppressors in spades. The rest is history as I like to say. As you can see through this article and the various sources I have given that the church gained its power and was established through military might and not divine providence. It was the union of the Roman Empire with the church (theocracy) that established it as the one and only religion. It's only serious threat came later when they faced off against Islam during the Crusades. To date Christianity and Islam are the two largest religions on the planet boasting the most adherents.
Note: For those of you interested in reading the original creed of Nicaea and its subsequent revision in 381ce please feel free to click here.