In its origin Christianity is most intimately connected with Judaism, the parent religion. The known world, however, in the time of Jesus was largely under Roman dominion.
[Paul] was an enormously talented and ambitious person. Without Paul, Christianity would not exist today. He is its founder; the one who built and popularized it. And he did it all in a 30 year period — approximately between the years 30 and 60 of the Common Era. By 100 CE, Christianity was already a very strong movement throughout the Roman world.
Christianity arose in a very tumultuous period when the Jewish world was caught under the heel of a brutal and immoral Roman culture, and was itself plagued by movements and groups which not only fractured it even further but often did so under the guise of saving the Jewish people.
During all his reign Theodosius took severe measures against the surviving remnants of paganism. In 388 a prefect was sent around Egypt, Syria, and Asia Minor for the purpose of destroying temples and breaking up pagan associations
As emperor, Theodosius dramatically expanded Christian power at the expense of other religions and state itself. On February 27, 380, he declared "Catholic Christianity" the only legitimate imperial religion (Potter, 1994). His efforts as emperor allowed a closer configuration between church and state.
Christianity was simply lumped together with other mystery religions of its day. To many in the first and second centuries AD, such Christian beliefs and rituals as monotheism, baptism, and the Eucharist (which sounded like cannibalism) were just as bizarre as those found in other cults.
From about 100 to 337, the Church in the Empire remained an illegal and persecuted sect. Still, the Church succeeded in adding to its numbers. It also developed a coherent body of theological and administrative opinion. By the early 4th century, the Christian faith had penetrated much of the world of the Roman Empire: it was the largest single religion within the Empire.
Christianity triumphed over these mystery religions after long conflict. This triumph may be attributed in part to the fact that Christianity took from its opponents their own weapons, and used them: the better elements of the mystery religions were transferred to the new religion.
Origen was the first truly philosophical thinker to turn his hand not only to a refutation of Gnosticism, but to offer an alternative Christian system that was more rigorous and philosophically respectable than the mythological speculations of the various Gnostic sects. Origen was also an astute critic of the pagan philosophy of his era, yet he also learned much from it, and adapted its most useful and edifying teachings to a grand elucidation of the Christian faith.
[From the Edict of Milan:] "When I, Constantine Augustus, as well as I, Licinius Augustus, fortunately met near Mediolanurn (Milan), and were considering everything that pertained to the public welfare and security, we thought, among other things which we saw would be for the good of many, those regulations pertaining to the reverence of the Divinity ought certainly to be made first, so that we might grant to the Christians and others full authority to observe that religion which each preferred..."
The controversy [over Arianism] seemed to have been brought to an end by the Council of Nicaea (ad 325), which condemned Arius and his teaching and issued a creed to safeguard orthodox Christian belief. This creed states that the Son is homoousion tō Patri (“of one substance with the Father”), thus declaring him to be all that the Father is: he is completely divine. In fact, however, this was only the beginning of a long-protracted dispute.