So I thought. These days, most Orthodox Jews would not consider people converted in a Conservative or Reform branch of Judaism jewish, right? But how do you know, as an Orthodox Jew, that you are 100 % jewish? I mean what if his or hers ancestor back back in a time converted in other branches of Judaism and he or she just do not know it? And you cannot prove it? Or there was no proper conversion whatsoever?

Is there some opinion dealing with this?

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    Reform Judaism is only about 200 years old. – Double AA Jul 12 '17 at 15:18
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    How is this different from "what if his or hers ancestor back back in a time was a gentile and he or she just do not know it?" – Double AA Jul 12 '17 at 15:20
  • Well, because there is no proof that his/her ancestor was converted or not or how. You know we had a WWII here so there is not a lot of information left anyway. – Elli Jul 12 '17 at 15:28
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    Sorry, I don't see how you've addressed my comments. If you can find a difference from the question I mention please edit your post to clarify. Seems to me like 99% of Jews can't prove all their ancestors were Jewish. – Double AA Jul 12 '17 at 15:39
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    Which question are you asking? The body of the question seems to ask "how do I know for certain that my ancestors are Jewish," in which case this is indeed a dupe. But the title asks something entirely different - what happens if someone thinks he's Jewish and finds out there was something wrong with his ancestor's conversion? – DonielF Jul 13 '17 at 5:01

There is a concept in the Talmud called "כל המשפחות בחזקת כשרות הן עומדות" (kidushin 76b opinion of the sages against R Meir). This means that every Jewish family has a kosher Jewish status-quo until there is reason to suspect otherwise, and a person is allowed to marry the family without any further investigations (בדיקות).

This is the accepted Jewish law. See Tur EE siman 2, which discusses in length how we should regard an unknown family and the likes. However, if the family is known to everyone there is no doubt that it has a kosher status-quo (according to some even R Meir agrees in such a case). According to the Tur himself even a Kohen may marry into a known family without investigating based on their חזקת כשרות.

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    But does this still apply? Back then they didn't have reform conversions. Maybe nowadays there is no longer a chazaka that everythings ok – Double AA Jul 13 '17 at 2:14
  • There were other branches which was not regarded as "Orthodox" or 100 % "clear". – Elli Jul 13 '17 at 18:27

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