Can a person be Christian and Jewish at the same time?

My grandparents were Jewish and left Germany in the 30s before the war. My father was raised Jewish, but the household was pretty secular for the most part. A few years before he married my mother he converted and is an evangelical protestant to this day. He still celebrates certain Jewish holidays, like Pover, and my family has already had a menorah in the window during Christmas. So, he has Jewish heritage, but no longer follows Judaism. Could a person like this be both, Jewish and Christian? I know that according to the Law of Return he cannot get Israeli citizenship because he converted, but I was just curious about this because there are many Jews that are secular, or even atheist, and they are still considered Jewish by the Jewish community because of their ethnicity, so why shouldn't someone that converts be considered Jewish too?