Once you are Jewish, you are always Jewish, no matter what you believe in the area of religion.
Jews are an unusual ethnoreligious group, that have had people debating to this day, what a Jew really is. What really isn't up for debate, however, is that while someone might have a distinct family lineage that seems to create their "jewishness", the reality is that no matter your family history, you're only a Jew if you're practicing Jewish Law. Torah is the Jews' covenant with God. Jesus is a Christian's covenant with God. Any members of a Christian church, that presumably accept Jesus as a messianic figure, are not Jews, regardless of their lineage.