I wasn't talking exclusively about ethnicity, but more about religious conviction
Then the word I think you were looking for was "Judaism," as in the faith instead of "Jew" as in the ethnicity.
But even that is something of a misnomer, as there was no single form of Judaism at the time. It was split into a number of very different sects with different beliefs.
Jewa became Christians too.
That's kind of you...
Christianity began in Jerusalem as a Jewish movement.
Everybody involved, from Jesus on down were Jewish.
Jesus explicitly stated that he was sent only to the lost sheep of Israel
Early Christianity was simply acceptance of Jesus fused to a full observance of the Law
James the Just, Jesus' brother and leader of the Jerusalem Church was a life-long Nazarite, who did not cut his hair or eat meat
The line between Pharisee and Christian was semi-permeable at first, which set the stage for the Jerusalem Council and the Apostolic Decree on circumcision.
Gentiles eventually became Christians too.