Jews: nationality determined by mother
Today, when most of the world's countries are more or less multinational, during study of interethnic relations issues experts ask what does "nationality" mean, is it political, social or biological concept.
In overwhelming majority of cases, child's nationality is determined by his father's, but for Jews, individuals born of Jewish mothers are concidered Jews. They have their own religious-philosophical explanation of this, written in the Torah. You don't have to read the entire Torah in order to understand "national question". You just have to read Book of Deuteronomy, which represents Moses' last speech. In this book, Moses essentially repeats the content of other books and gives instructions and prophecies to sons of Israel.
"When the Lord your God brings you into the land that you are entering to take possession of it, and clears away many nations before you, the Hittites, the Girgashites, the Amorites, the Canaanites, the Perizzites, the Hivites, and the Jebusites, seven nations more numerous and mightier than you, and when the Lord your God gives them over to you, and you defeat them, then you must devote them to complete destruction. You shall make no covenant with them and show no mercy to them. You shall not intermarry with them, giving your daughters to their sons or taking their daughters for your sons, for they would turn away your sons from following me, to serve other gods. Then the anger of the Lord would be kindled against you, and he would destroy you quickly," Moses wrote.
This is official Jewish interpretation. Meanwhile, esoteric branch of Judaism - Kabbalah, which was especially popular in the 13th century - sees not only religious, but also mystical nature of this problem. Its followers believe that at the time of conception soul of a Jewish woman attracts only Jewish soul, and that's why nationality must be determined based on mother's nationality.
There are also more prosaic explanations.
Sociologists claim that since mother is mainly responsible for upbringing of the child, child learns about cultural values from her. Wise Jews rightly believe that cultural identity mainly defines nationality.
Genetics have their own view on this issue. Percentage of men who have doubts about paternity has always been relatively high. Modern studies show that about 10% of children born in a marriage don't have genetic relation with a father, specified in birth certificate. DNA test became very popular. But even in the distant past, not all fathers could say with certainty that their children were their own, but \mothers never have any doubts.
Finally, it's no secret for anyone that both body and soul of child are formed in the womb, and from all women of different nationalities, God gave Jewish woman saving mission of gaving birth to Jesus. Althogh there are still debates about nationality of both Jesus himself and Mary, Jews' nationality is determined by mother.