Obviously there is a difference between crossing lines that separate Orthodox , Conservative , and Reform Judaism, and crossing lines that separate Judaism and Christianity . Jewish life is undergoing constant change in America . Families have roots in many types of Jewish congregations. The difference in observance is getting less, but between Jews an Christians , although social contacts may increase, the basic theological difference is unshaken. If, as we must assume, belonging to a congregation means accepting its teaching, then a Jew cannot belong to a Christian congregation. Christian congregations are trinitarian and Jewish law prohibits a Jew from adding other divine personages to God in his prayers. Likewise, Christians cannot be part of a Jewish religious community and still be Christians , because they would then have to deny the role of Jesus as Christ .
But it happens that in our present social conditions people contribute to many congregations they do not accept. This is due to our praiseworthy American mood of interdenominational charity.
It is therefore quite conceivable that a Christian would want to make a donation to a synagogue, and this is quite in accordance with Jewish law(Reform Jewish Practice, 11, 45ff). He might consider his dues as merely a charitable contribution. This is, of course, a possibility, but it certainly implies a confusion of theologies.
However, since the case you mention involves the Christian 's making himself eligible to be married by a rabbi, then he means his membership in the Jewish congregation to be an evidence of conversion. Conversion is something absolute in the eyes of Jewish law. It is so absolute that the law in the Talmud (as codified in the Shulhan Arukh, Yoreh Deah 268:9) says that a proselyte is like a newborn babe. He has not even any relatives left in his former life. This is, of course, an overstatement which the law itself modifies. But it means that the division must be clean-cut.(By the way, I am
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