Yes, the first Christians USED to be Jews. That is very different from what you said: that Jews are Jews when they believe in Jesus.Why? The Bible. Many Christians read and study their Bible. Many Jews believe Jesus is the Messiah. Tell me, were the very first Christians all Jews? Were the disciples of Jesus Jews?
I do not believe in Jesus, as 99% of my fellow Jews do not. You’re saying that we aren’t Jewish?!