What is a Jew that converts to christianity?

Answer 1

There are no statistics available for this, because once a Jew leaves Judaism, he or she usually does not identify as a Jew any longer.

The exception is a religious denomination called Messianic Judaism, however a recent survey shows that more than 70% of the members of these religions were never Jewish in the first place.

Jewish-Christian Answer

A recent University study shows that at least 20% of the population of Spain,

say 9 million people have a mother with Christian Jewish genealogy.

that would make of Spain the country with the largest community in the world

of Christian Jews.

Answer 2

Such a person will no longer be a Jew; he will be a Christian.