How can one tell if one needs to convert or if one is already Jewish?

Would one's or one's ancestors' conversion to another religion or their lack of knowledge about Judaism change anything?

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According to traditional Judaism, you are Jewish if and only if you yourself have validly converted to Judaism OR your birth mother was Jewish at the time of your birth (Shulchan Arukh Even HaEzer 7:17 and 8:5, Yoreh De'ah 268:6). To determine if your mother was Jewish apply the same rules: either she herself converted to Judaism OR her birth mother was Jewish at the time of her birth. This process recurs at least back to the Revelation at Sinai. NOTE that in order to prove any of these claims you may need to provide more than just hearsay or oral family tradition. If you have a particular concern regarding your own family history, you should speak to your local rabbi for guidance.

As far as conversion to another religion or other lack of Jewish practice, doing so would not take away anyone's status as a Jew. It may create certain obligations related to repentance and losses of certain rights prior to said repentance (see Can a Jew lose their Jewish Status?) but as far as the previous paragraph is concerned, the person is still Jewish.

See this article for more details about the rules and history of Jewish status.

  • if I remember correctly there is a shitta in the Shuclahan Aruch which says that a Jew who converts to another religion needs to convert again,I think in hilchos yibum – sam Jan 20 '15 at 17:28
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    @sam You are probably thinking of the rama in #3 of the linked answer which I also cite here judaism.stackexchange.com/a/48507/759 – Double AA Jan 20 '15 at 17:28
  • This is what I was referring to beta.hebrewbooks.org/tursa.aspx?a=eh_x8273 , I remember learning that the Geonim held they are not Jewish anymore,but the Rambam as you quote doesn't go with it,but we see they have an inbetween state ,halacha 4. It's more of a chiddush than losing yerusha since yibum is mitzvah – sam Jan 20 '15 at 17:56
  • @sam Ah, right. I thought that's more about losing Achvah than losing Jewishness? I don't think that Deah would hold that the Mumar's kids would need Gerut. – Double AA Jan 20 '15 at 18:08
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    Why the downvote? – Double AA Feb 18 '15 at 5:27

Sorry to spoil the party. DoubleAA's answer talks about "de-jure" Jews, or questionable situations. In practice situation is different.

Whoever is unanimously and publicly accepted as a Jew is a Jew de-facto! As Kiddushin 73a, Rambam Melachim and Shu"A in Even Haezer 2, 5: "משפחה שנטמעה - נטמעה" (Family that assimilated - assimilated.)

I bet, 90% of families (incl myself) can not trace their ascendants back more than 3-4 generations, but in fact it does not matter, as long as those 3 generation openly practiced Judaism we hold their descendants as Kosher Jews de-facto.

As a result, any Reform or other non-Orthodox convert that will see his grandkids observing Torah, will not suffer anyone questioning their Jewishness.

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    DoubleAA wasnt only talking about Jews of questionable status (although the question is most relevant to them, since by definition, if someone's sure about his status, there is no question. || He wrote: to prove any of these claims you may need to provide more than just hearsay or oral family tradition. If you have a particular concern regarding your own family history, you should speak to your local rabbi for guidance. That is, family tradition may be enough. If you have a question, consult a rabbi. Nothing controversial about this, nor contradictory to your post. – mevaqesh Nov 22 '17 at 19:51
  • This is circular and hence circular. If they are unanimously accepted as a Jew, then the question doesn't start. The question in the OP was about someone who has questions and doubts. – Double AA Nov 22 '17 at 20:17
  • If the Gemmora says that what can I add? משפחה שנטמעה נטמעה. I can (seemingly) "prove" my Jewishness for the last 4 generations but 2-3- hundred years back? I have no idea who my ancestors were. Can it be that my grand-grand-grand mother was a shiktze? a convert? No idea. – Al Berko Nov 23 '17 at 14:58
  • @al berko first, spoil the party line is a little much. also, i believe you did not really address the issue at hand. – heshy Jan 23 at 2:34

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