You appear to be conflating religion and ethnicity. Muslim is one who subscribes to the Islamic religion. A Christian is one who subscribes to the Christian religion. However, this is unlike with Jews: a person can be a Jew if he is only ethnically Jewish (born of Jewish lineage). You can be both Jewish ethnically and Jewish religiously, or you can be only Jewish ethnically. (You can't be Jewish only religiously - if you are accepted as religiously Jewish, you are automatically considered to become a part of the ethnicity. And there are genetic markers for being Jewish ethnically: most notable is probably the Cohen y-chromosome haplotype, which can at least designate Semitic origin.) There is no Muslim or Christian equivalent because those religions span a multitude of cultures, ethnicities, and regions of the world. In fact, the most populous Muslim country is not at all Arabic.