Then they are no longer Jews.
What are they before they convert, then ?

Do you think all of them instantly convert? It usually takes a long time for someone to convert.
Considering how much anti-Christian propaganda they are fed by their fellows, it can be quite difficult to question all of this nonsense.
I remember from testimonies they are told the Bible is antisemitic propaganda or so on so most of them never even think to open a Bible and verify if it's true.
You know very well most humans on the planet fall victim to one sort of propaganda or the other.