Becoming a Jew is not just a one-time declaration of faith but rather an evolutionary process that culminates in the adoption of a new cultural, national, and even historical identity. Conversion to Judaism requires serious study, active participation in Jewish holiday and lifecycle events, and, finally, a commitment to Jewish practice that is actualized by a rabbinically prescribed ritual.
Even the talmudic rabbis understood that a transformation of such magnitude as conversion had to be motivated by sincere conviction, what they termed "for the sake of Heaven." In fact, the rabbis did not accept conversions to accommodate another's wishes or of to achieve social or economic gain. Many contemporary rabbis, however, allow conversions for the sake of marriage, holding that many such converts eventually evolve into sincere Jews.
The history of conversion reflects the treatment of Jews by surrounding cultures, moving between active proselytizing and passive acceptance of converts as the majority culture becomes more and less hospitable to Jews. In the biblical period outsiders joined the Israelite nation via assimilation; the beginning of formal conversion history awaited the evolution of the nationalist God of the Torah to the universal God of the prophets as well as the move from the sacrificial Temple cult to the portable religion of prayer and study developed by the rabbis.
With a God who could now be worshipped outside the land of Israel, the stage was set for a period of more active proselytizing that culminated in a Roman empire that was 10 percent Jewish by the beginning of the Christian era.
Later, when Christian and then Moslem authorities made conversion to Judaism illegal, Jews turned inward, focusing on careful observance of God's law and left the initiation of the conversion process to the potential converts themselves. The attitude of mistrust and even dissuasion of potential converts from this period became the Jewish "tradition" when enshrined in the medieval Jewish law codes. The emancipation set the stage for a reconsideration of the appropriate boundaries between Jews and non-Jews.