After the martyrdom of Stephen, the Jewish church moved out beyond Jerusalem into Samaria, the Galilee, and further north and east, even into the region of Damascus. However, there also remained a large group at Jerusalem. The apostles were among those that stayed and the center of authority remained there, as well. Even after the martyrdom of James, Yeshua's half-brother and the leader of the Jerusalem church, by the temple authorities in 62 A.D., many of the Jewish believers remained.
However, before the destruction of Jerusalem and the Temple in 70 A.D. the Jewish believers fled. Eusebius (325 A.D.) wrote:
"For when the city was about to be captured and sacked by the Romans, all the disciples were warned beforehand by an angel to remove from the city, doomed as it was to utter destruction. On migrating from it they settled at Pella, the town already indicated, across the Jordan. It is said to belong to Decapolis (de Mens. et Pond., 15)."
Many Jews died in the Jewish War with Rome and so the fleeing Jewish Christians were considered traitors. This led to a severe rift between the believing Jews and the non-believing Jews. Although, many returned to Jerusalem after the war, the relationship between the two groups was strained even more than it had been.
Finally, the remaining Jewish believers left Jerusalem either during or after the Second Jewish War (132-135 A.D.) when the Emperor Hadrian banned all Jews from Jerusalem, which was renamed Aelia Capitolina.
Meanwhile, Paul and eventually others, had begun many Gentile Christian churches. By the time of the Second Jewish War, Rome took a decidely anti-Jewish position, furthering the desire for the Gentiles to be separated from anything Jewish. Also, as the influence of the Jerusalem church faded for the Gentiles, Torah observance began to fade also.
For a long time Gentile Christian churches existed side by side with Jewish Christian churches, although, the Jewish group did not call themselves Christian, but rather Nazarene, since they continued to believe themselves a sect of Judaism. But finally, in the fourth century, the Gentile Christian church began to view the Nazarenes as heretical, based solely on their continued Torah observance. Epiphanius (310-403 A.D.) said this:
"We shall now especially consider heretics who call themselves Nazarenes; they are mainly Jews and nothing else. They make use not only of the New Testament, but they also use in a way the Old Testament of the Jews. For they do not forbid the books of the Law, the Prophets, and the Writings...so that they are approved of by the Jews, from whom the Nazarenes do not differ in anything, and they do profess all the dogmas pertaining to the prescriptions of the Law and the customs of the Jews, except they believe in Messiah. They preach that there is but One God and His Son Yahushua. They are learned in the Hebrew language, for they, like the Jews, read the whole Law, then the Prophets...They differ from the Jews because they believe in Messiah, and from the Christians in that they are to this day bound to the Jewish rites such as circumcision, the Sabbath and other ceremonies.."
About that time the Nazarenes ceased to exist as a separate sect, finding no common ground really with the Jews or Gentile Christians, although pockets of believers practicing at least part of the Torah have existed throughout history. It was the Gentile Christian church, for the most part, that was the only surviving body. The Jewish church was simply overwhelmed by numbers and by historical events, but it never veered from its staunch support of Torah obervance, nor did it allow the paganism that the Gentile church allowed or tolerated as Christianity became fashionable, and eventually the approved religion of the Roman Empire. The reason for this tolerance was to make conversion from paganism easier. Many pagan traditions were allowed and supposedly Christianized.
So, this is what happened to the Jewish church and how Christianity became so non-Jewish, so different from the Jewish church that began it. Although, this may seem to be a sad outcome of history, God has allowed it, and there does seem to be a resurgence of Torah observant believers in Yeshua.