We know from Scripture that the Messianic Community in Jerusalem (and Lod, the Sharon, Joppa and the Galilee) was thriving in the first decades after the outpouring of the Holy Spirit in Jerusalem. When Paul comes back to Jerusalem to greet the apostles, they report to him:
“On hearing it, they praised God; but they also said to him, ‘You see, brother, how many tens of thousands of believers there are among the Judeans, and they are all zealots for the Torah’ (Acts 21:20 CJB).
There are two interesting points worth noting. First, they are Torah-honoring Jewish believers. This does not mean that they necessarily followed all the traditions of Pharisaical Judaism, but that they suddenly found deep meaning in the laws that they previously only kept out of religious guilt. This is reported to Paul as a good thing. There is no hint that they are moving away from Torah or their Jewishness, but closer.
Secondly, many translations use the English thousands for the Greek myriads. However a myriad is 10,000, so myriads plural, as is used in this verse, is correctly tens of thousands!
The congregation continued to grow under the leadership of Jacob (there were no first century Jews named James) the brother of Yeshua until the destruction of Jerusalem in 70 CE (some believe that Jacob died earlier).
Flee to the Mountains
Now this is where things start to change drastically for the Jewish believers in Israel. Yeshua had told His disciples less than 40 years before that when they see the armies surrounding Jerusalem they should flee to the mountains.
“But when you see Jerusalem surrounded by armies, then know that its desolation is at hand. Then let those who are in Judea flee to the mountains. Let those who are in the middle of her depart. Let those who are in the country not enter therein” (Luke 21:20-21, WEB).
“The Great Revolt” took place in 66 CE when the Jews rebelled against Roman rule. The Romans responded by leveling the city and destroying the Temple in 70 CE. As many as one million Jews died all over Israel.