I apologize for quoting myself here, but there were a couple things I wanted to add since (in response to Master50's interesting historical post):
The interesting thing about the highlighted bit is that there were serious discussions among Jews as to whether or not Jesus teachings were at all meant for the pagans. Peter didn't think so.
Saul, later St. Paul, however did and began his missionary work among the gentile peoples to convert them. At a certain point they, the pagans, transformed the historical figure of the Jew, Jesus, into the supernatural Chritos (Greek, a gentile language, for Savior) or Christ and from which the religion got its name. They provided Christ with a Hellenized and classical imagery and they invented Christianity, as such, as a distinct religion from Judaism. It was a happy fiction that promised salvation, but a fiction nonetheless in my way of thinking. Not having themselves been Jews, the pagans weren't interested at all in becoming Jewish, but something else; while St. Paul realized that making the gentile converts abide by the strict dietary rules of the Jews was not doable and so pointless. So Paul emphasized to the pagans Christianity's identity as being almost exclusively wrapped up in his personal impassioned, and fanatical, belief in the resurrection of Christ (something which appeared less decisive, or at any rate less fundamental, to Jews who believed that Jesus was the Messiah), rather than the old proscriptions of the Torah. This proved to be a propitious strategy, St Paul's teachings to the gentiles, for the pagans were already quite familiar with tales of gods being reborn or resurrected in their mythology.
In other words Christianity emerged from the classical traditions of the gentile peoples and, as such, got its autonomous non-Jewish identity from the pagans in the antique world, which is the same identity that the orthodox Christians of the Latin/Roman Catholic and Greek churches (the oldest surviving forms of the religion, of which there were many variants in antiquity - Arian, Donatist, Monophysite, Gnostic, etc.) have inherited and know it as till this very day. And it was they, the pagans, who also fixed its religious calendar: with dates like Dec. 25 being a recycled sun god birthday, was now for the Christos, while Easter recalled the pagan springtime resurrection celebrations of deities like Attis that were symbolically connected to the rebirth of nature itself, etc. All the protestant Christian sects came much, much later, of course, though conventionally still use these orthodox dates to celebrate the birth and resurrection of Christ.
Constantine himself, the Roman emperor who first legalized the sect, was initially devoted more to Sol Invictus (another pagan sun god whose birthday was Dec. 25) and his Persian counterpart Mithras (also born on Dec. 25, the old winter solstice), than the Christ, even after his “vision.” And he seems to have considered the worship of Christ in terms of Babylonian/Oriental sun god worship, to the extent that some scholars believe that had Constantine wholeheartedly embraced say Mithraism, we'd all be worshiping Mithras today instead of Christ in a manner of speaking.
The process, at least in the arts and culture, began before Constantine though: let us say about a century before in the Ancient Roman World, while the council of orthodox bishops at Nicaea (325), called into order by the emperor, tried to establish what the acceptable theology (hence orthodoxy as opposed to heresy) was among the Christian Churches of the Roman Empire, but failed. Indeed Jewish Christians, who of course would not have seen themselves as such, but Jews who simply worshiped Jesus as the Messiah, continued to practice their faith in Palestine long after the hegemony of orthodoxy in the Eastern and Western Roman Empires was established after 325. They would have though eventually been persecuted, along with all the other non-orthodox forms of Christianity, by the orthodox bishops and their followers in the late IV and V centuries under the support of the last Roman emperors in a colossal bid to stamp out, once and for all, the so called heretical Christian faiths. But even this failed.
Arian Christianity, for example, which taught that God and the Son were not of the same substance and therefore the Son was a lesser god, was still practiced by the Goths and other barbarian tribes of Germanic origin who caused the fall of Rome and the West by the late V century, and would not be conquered religiously until Pope Gregory the Great's (590-604) missionary priests converted them all to Catholicism. For the first time all of Western Christianity, among the Germanic peoples and Latin populations of the former Roman Empire alike, would look at the bishop of Rome as their universal (catholic) spiritual authority. Hence Gregory the Great transformed Rome and Europe as well, by giving the papacy its medieval identity, which would even have political consequences throughout the rest of the Middle Ages.
Now the Jewish Christians to escape persecution by the orthodox church continued to practice their faith in secrecy, and, to evade the Byzantine authorities (VI century), probably, or so it has been convincingly argued by the period scholars, left Palestine and brought their branch of "Christianity" to the Arabian Peninsula, where, mixing with the native, nomadic Arab tribes would eventually plant the seeds of a new religion: Islam. We don't know all the details about this, though the historical contours are fairly clear.
Finally, what's also interesting in this religious evolution from paganism to Christianity and from Judaism-"Christianity"-Islam, is that one of the apocryphal gospels, that of Judas (that is contemporary with the 4 canonical ones), which was probably widely circulated among the Jewish followers of Jesus who eventually arrived in the ancient Arab world: it consists of a series of secret verses, messages of a hidden Truth meant for Judas alone and supposedly spoken to him by Jesus himself after the resurrection, that are very much in synch with the later style and import of the Koran. Could it be, consequently, that the sacred book of Islam arose form such apocryphal Early Christian texts that did not survive within orthodox Christianity, but did among the persecuted Jews who worshiped Jesus and who arrived in the Arabian desert by the early Byzantine period? This is precisely one academic school of thought.
This is why, to me, religion is a historical, and consequently man made, construction, which, like all historical processes, needs time to become fully elaborated into the various identities and ideologies it eventually assumes - that is before fading into, as the pagan gods once did (another part of the historical process), oblivion. Then comes the inevitable establishment of a religious hierarchy and the theological interpretations and exegesis of the original message and what was subsequently said and written about it, which, since they have often been in direct conflict and opposition to one another, among the respective camps, have lead to the many violent conflicts and bloodshed we are all too historically familiar with. In short the establishment of rigorous dogmas and behavior codes that are nothing less than that hierarchical establishment's attempt to bend society and conform it to what it says is the Truth, for which no alternatives can be contemplated. They take something ineffable and mysterious and twist it into an often demented system of mass repression. While frequently, because of this, the dogmas are an abomination of the original message, even if it seems that religion simply can't make due without them.
The not infrequent dreadful consequences for such intractable behavior and fanaticism of the religious establishments, have been, unfortunately, plainly visible to everyone. In any case, since alot of the religious establishment is conservative and, in this sense, often behaves much like the political conservative establishment: no doubt this is why, personally, I have long been turned off by them both and have avoided each like the plaque. As if in my formative years I developed antibodies against a disease, which have only grown stronger as a defense since I've gotten older.