The friends of Jesus, or friends of his friends, were originally a bit unconventional, disliked and regarded by various authorities of their time as troublemakers—but at first, quite possibly, they did not think they were starting a new religion.
In the first 30 to 40 years after Jesus' execution they must have run about like scared rabbits. There is little trace of them until the 60s and 70s of the first century.
Before that, they were trying to make sense of what had happened, why he had died and whether they had actually seen Jesus alive after death—if so, what that meant.
The Pentecost experience—the dove, the tongues of fire, the strange warming, a sudden emergence of paranormal gifts (including the nerve to preach openly about the executed woodworker they called "Lord")—wasn't just a matter of feeling spiritually "high" 50 days after Easter.
They were Jews, albeit with some uncommon ideas. To them Pentecost (Gk. Pentekoste, "the fiftieth [day]") was the Feast of Weeks, celebrated today in Judaism as Shavuot. It recalls Moses receiving the Law on Sinai.
Wandering in the desert of despondency, afraid of Roman soldiers and the rabbinical authorities alike, these plain folks who had followed the Galilean experienced something akin to the reception of a new Law. They suddenly saw a new divine reign being born and gained the conviction that a coming upheaval would transform the established human order into a civilization of love.
Initially, they were treated as yet another annoying little group, one of many in the bazaar of religious opinions that was the Judaism of the time.
Then all hell broke loose. A revolt erupted against Roman rule in Judea. It ran from the year 66 of our era to 70.
The contemporary Jewish historian Flavius Josephus called it The Jewish War, a war of attrition by starvation and blockade by the most superpower of the day, waged in one of the empire's marginal provinces that was the homeland of a Jewish populace that stubbornly clung to its monotheistic way of life.
The year 70 marked the defeat of the rebels, the razing of Jerusalem, including Solomon's temple, of which a portion remains today as a site of prayer and remembrance known as the Wailing Wall.
As if the symbolic destruction was not enough, Jews were expelled from their homeland. Thousands were marched in chains to Rome (where they were pressed to build, among other things, the Coliseum). Most others escaped, bringing about the dispersion of the Jewish nation to all corners of the earth.
For Jews it was a disaster not unlike the plunder and captivity under the Babylonians centuries earlier. Among the stricken Jews was the little band of "Jesusites," who some called Nazarenes.
Unfortunately for the Nazarenes, in the eyes of the rabbis the time for temporizing and the luxury of debates on fine points had pretty much ended. At least for a while. The religious authorities of the Jewish nation did everything in its power to establish a fixed, concentrated version of Judaism.
The Jewish faith that had been rooted in the Hebrew Scriptures* grew new roots in the Talmud, the collection of rabbinical interpretations that have guided observant day-to-day Jewish life to the present.
The Nazarenes were expelled from most synagogues and virulent polemics called them "apostates" or, more the popular insult Kristianoi (a Greek street term you might think of as "Christies," much as today's Unification Church members are often called "Moonies" disparagingly after the name of their religion's founder).
Lest you doubt the animosity involved, consider polemical pro-rabbinical claims that Jesus' mother Myriam was really a whore and Jesus the son of a Roman who raped her. Conversely, note the often polemical use of the term "the Jews" in John's gospel, written well after the destruction of Jerusalem, when the Christians, still overwhelmingly Jewish, were trading barbs with their Talmudic-minded Jewish brethren.
Two factors—which did not necessarily arise in the order chronicled here—combined to solidify the rift between Jesus followers and other Jews.
First, the Roman authorities had immense trouble distinguishing between Jews who liked Jesus and those who did not. The Romans were ill-disposed to all Jews after the revolt and were not in a mood to tolerate street disputes between Jews—persecution of Jesus believers was probably at first anti-Semitic, rather than anti-Christian.
Second, a Jewish rabbinical student who had grown up in the Greek-speaking Asia Minor city of Tarsus dramatically shifted alliances from the rabbinical party to the pro-Jesus party. He became one of Christianity's most effective paladins, preachers, popularizers and propagandists. We know him as St. Paul.
There were other aspects of the lifestyle of the early Christian communities that set them apart—many are noted in the Acts of the Apostles—and I shall deal with them later.
For the moment, let's recall—centuries of Christian anti-Semitism and Jewish anti-Christian critique notwithstanding—that Christianity's separation from Judaism was, at heart, a Jewish quarrel launched two millennia ago.
* See the second paragraph of Story of the Faith 16 - Prophets.