I thought this was interesting....therefore...I will ask a question.  Do Jews really believe that a Jew who becomes a "christian" is no longer a JEW?  Peace and blessing is my prayer. G.

A Jew by birth or proper conversion is always and forever a Jew.  The key issue of your question is  the individual's relationship to the people of Israel.

A Jew who assimilates to Christianity is expelled from the community. They are "cut off from the people." Our scriptures have a few stories of individuals who were cut off for a variety of reasons, not just adopting different religions.  

Even though they are 'cut off',  their identity as Jews can not be changed. And some Jews are convicted that the actions of each individual Jew impacts the whole.  G-d certainly has dealt with us as a nation.  Observant Jews feel an obligation to help other Jews be good Jews.  So, emotionally, assimilation, it is like having your son or daughter reject not just you, but your nation as well.

Hope I answered your question,
Shalom