Would the earliest followers of Jesus recognize the religion of Christianity practiced by 2.2 billion people today? While we cannot be certain about what the original Christians believed, many scholars argue the religion drastically changed in order to establish and retain a large non-Jewish following.
According to the Acts of the Apostles
After the death and resurrection of Jesus, the apostles set out to preach his words and his story to the Jewish  population. To them, Christianity (a term they did not use) was entirely Jewish, and all adherents still had to follow the laws and customs of Judaism set about in the Torah (Old Testament). To the Jewish authorities, however, this new sect was sacrilegious and they set out to punish the Christians for their blasphemy. One of the authorities, Saul, had a religious vision of Jesus and was inspired to preach his message to all who would listen. Renamed Paul (the Apostle), he traveled throughout the Roman Empire converting gentiles (non-Jews) to this new religion. Paul was very successful in establishing a number of Christian communities, but angered the Jewish Christians because he insisted much of the laws of the Old Testament no longer applied to the followers of Jesus. This, according to the New Testament, was the sole difference between the beliefs of the original Jewish Christians and the Pauline sect of Christianity.
What Did the Original Christians Actually Believe?
After hundreds of years of competition for adherents, Pauline Christianity surpassed both Jewish and Gnostic (mystical Christianity) sects to become the sole survivor. There are several reasons for Paul's success:
- Pauline Christianity was much more organized and had a more palatable message.
- The Jewish-Christian community was almost entirely snuffed out in 135 CE when the Romans killed, enslaved, and banished all Jews living in Palestine after the Bar Kochba revolt.
- After gaining a strong foothold in the eastern Mediterranean, Gnostic Christianity was driven to extinction by Pauline Christians who considered them heretics.
As the maxim goes: "history is written by the victors," and it was Pauline Christians, after all, who compiled the New Testament books. Thus, there is reason to believe the Bible doesn't present a true picture of what the original Jewish Christians believed. However, in several Pauline Christian writings regarding heretical sects, there are references to various Jewish-Christian groups such as the Ebionites. These Jewish-Christians had very different beliefs from those espoused in the New Testament, and there are some scholars who consider them to be more authentic to the original form of Christianity. As religious historian James Tabor put it:
"They neither worshiped nor divinized Jesus as the Son of God, or as a Dying-and-Rising Savior, who died for the sins of humankind. They practiced no ritual of baptism into Christ, nor did they celebrate a sacred meal equated with 'eating the body and drinking the blood' of Christ as a guarantee of eternal life. Their message was wholly focused around their expectations that the kingdom of God had drawn near, as proclaimed by John the Baptizer and Jesus, and that very soon God would intervene in human history to bring about his righteous rule of peace and justice among all nations."
Before understanding how Paul and his followers may have altered the original Christian message, it is important to put his mission in context. His "target market" was pagans of numerous nationalities and socioeconomic backgrounds living in metropolitan Roman cities far away from their cultures of origin. In order to reach these people, he needed to strip away the strange "Jewishness" of the original sect of Christianity, and find ways of resonating with these very different people. As he wrote in 1 Corinthians 9:22-23, "I have become all things to all men, so that I may by all means save some. I do all things for the sake of the gospel, so that I may become a fellow partaker of it."
The Gospel of Love
In the book "The Evolution of God," author Robert Wright argued that the idea of love spanning cultural boundaries was not a part of Jesus' original message. He pointed to the story told in Matthew 15:21-28 where a gentile woman asked Jesus to help heal her daughter. Initially, Jesus dismissed her saying, "It is not right to take the children's bread and toss it to the dogs." Eventually, he helped her after she stated, "Even the dogs eat the crumbs that fall from their master's table." While Jesus may have had little respect for people from different cultures, Paul was trying to form Christian communities among individuals from very different backgrounds. Thus, he instructed his followers that all were equal in the eyes of God. This message of love and inclusion was a good sales pitch for those described by classicist E. R. Dodds as the many "rootless inhabitants of the great cities, the urbanized tribesman, the peasant come to town in search of work, the demobilized soldier, the renter ruined by inflation, and the manumitted slave." This message of love and community would have been very attractive to the downtrodden and lonely. Not to mention, it cohered many members into a tight knit community who were motivated to share their message of love with everyone with which they came into contact. In short, love and compassion became a major element of Pauline Christianity because it was useful in attracting and retaining members.
Many scholars believe that Paul and his followers borrowed ideas and rituals from Pagan religions in order to appear more attractive to new members. This was not uncommon of other religions as well, since early Christians complained the Cult of Mithras borrowed the Eucharist from their practices. In fact, there are several parallels between Mithraism and Christianity, including the shared birthday of Jesus and Mithras on December 25th, though it is not clear which religion started which practice in all cases. There were also parallels with the Egyptian Cult of Osiris who drank beer and ate bread in rituals similar to the Christian Eucharist.  They also performed baptisms, and celebrated the miraculous birth as well as the death and resurrection of Osiris.
In traditional Judaism there is no well-defined concept of an afterlife, and Heaven was merely a term used to describe the sky. The early Christians did not believe in the modern concept of Heaven either. In fact, they believed the Kingdom of Heaven was the world inhabited by God and the angels, which would shortly make its way to earth. Once here, angels would cleanse the land of sin and sinners, and the righteous would live in a paradise alongside their resurrected dead relatives. Early Christians believed this would occur within their lifetime, but when it never happened they needed to find a way to reinterpret this concept of Heaven. It's not clear what led to Heaven becoming synonymous with the afterlife, but the modern concept has parallels with other religions known to the early Christians. For example, the Cult of Osiris believed the dead would come before Osiris who would judge them based on their righteousness. If he deemed them to be pure of heart, they would live in eternal bliss with the gods for eternity. If he considered them impure, they would be devoured by the monster Amemait.
Would the early Christians recognize the religion in its modern form? Probably not. Modern beliefs and rituals would likely appear very Pagan, and the deification of Jesus would be considered blasphemous. In addition, when considering the comfort of a blissful afterlife and the culture of compassion which makes modern Christianity so appealing, early Christians would again be perplexed. These were likely later inventions added to the religion to attract and retain membership.
Fascinating documentary discussing the parallels between Christianity and other religions
Great article on the history of Christianity
Great article on Pauline Christianity
Good article on the history of the Ebionites