A Jew is a person who according to Judaism is subject to the covenant that God made with Abraham. Different Jewish sects have different rules to deterrmine who is a Jew. By no means all Jews are believing Jews or practising Jews.
Jews and Christianity
Some evangelical Christians set out to convert Jews to Christianity and some Jewish converts to Christianity practise Christianity as other Christians do. By contrast Messianic Jews believe that Jesus was the Messiah, that makes them a type of Christian. Still Messianic Jews see themselves as Jews, they have rabbis  and some keep Jewish holidays with different meanings. Non-Messianic Jews see them as schismatic and non-practising Jews. 
Jews tend to be well educated. Many see for themselves that religion doesn't make sense and become Freethinkers.