Do you have to believe that someone is Jewish?

Question

If you are told by another person, that Mr. 'X' is Jewish do you have to believe it, even if nothing Mr. 'X' does shows any indication that he is Jewish? Do you have to treat Mr. 'X' as a Jew? Would it matter if Mr. 'X' himself said he is Jewish?

Answer 1

This is a very complicated issue. I'll start with the point of if Mr. X himself says he is Jewish.

The Talmud in Pesachim 3b has the story of a non-Jew who was given to eat from the Korban Pesach when he said he was Jewish. Tosefos there writes (s.v. ואנא):

מכאן אין ראיה שנאמין לכל הבא לפנינו ואומר ישראל אני דשאני הכא דרוב ישראל היו ואזלינן בתר רובא אך יש להביא ראיה מפרק החולץ (דף מה. ושם) דאמר ליה זיל גלי או נסיב בת מינך וכן מההוא דאתא לקמיה דרבי יהודה ואמר נתגיירתי ביני לבין עצמי אמר ליה נאמן אתה לפסול את עצמך ואי אתה נאמן לפסול את בניך והיינו משום דמצי למימר ישראל אני

From here ther is no proof that someone who says they are a Jew is believed, as here there was a majority of Jews. However, proof could be brought from Yevamos 45a [in which someone who claimed to be the son of a non-Jew with a Jewess] was told to go somewhere that they didn't recognize him and marry a Jew, and from the case of a man who said he converted, and Rebbi Yehuda said he was not believed to invalidate his son, the reason being because he would be believed [מגו] to say that he is a born Jew.

Rabbeinu Tam in Yevamos 47a says that there is a rov (majority) that most people who claim they are Jewish, are (and therefore proof can even be brought from the Gemara in Pesachim). The Rosh in Yevamos 4:37 cites this opinion as well, and Shulchan Aruch Y.D. 268:10 codifies according to this reasoning (Shach there).

R' Moshe Feinstein (Igros Moshe E.H. 1:6) wrote that even if the person does not "act" Jewish, just claims as much, they are believed:

ובדבר מה שאין ראיה על אם אשתו שהיתה יהודית רק מה שהיא אמרה לבתה ולו שהיא יהודית, נראה דלתוס' ורא"ש יבמות דף מ"ז דבאדם שבא ואמר ישראל אני נאמן משום דרוב הבאין לפנינו בתורת יהדות ישראל הם, אינו דוקא שראינום שומרי תורה ומצות אלא אף על אמירתם לבד שהוא ישראל הוו הרוב משום דעכו"ם לא יאמר על עצמם שהם ישראלים בכל המדינות שידוע שרובא דרובא דהעכו"ם ואולי כולם הם שונאי ישראל ומתביישים לומר שהם ישראלים, וכ"ש באשה שהיא נשואה לעכו"ם שאם היא ג"כ עכו"ם לא תאמר על עצמה שהיא ישראלית ויש להאמינה למה שאמרה שהיא ישראלית

Paraphrased:

If we have no proof that she is Jewish, just that she says she is, it seems that the words of Tosefos and the Rosh is not specifically by those who keep Torah and Mitzvos, but by anyone who says they are Jewish...

However, if there are previously established assumptions (chazaka) that a person is not Jewish, then they are not believed - Tosefos Pesachim 3a. R' Moshe (E.H. 1:8) asserts that even the Rambam (who makes no mention of the rov or מגו qualification) would still not accept it if there was a chazaka to the contrary.

If someone tells you Mr. X is Jewish, I imagine it would depend on how they know. If Mr. X told them, and the person telling you is Halachically trustworthy, then you would believe them. If they are just assuming, then I don't see why they would be believed. If they have circumstantial evidence, it would depend on the level of evidence. For example, R' Moshe rules (E.H. 2:10) that someone with a Jewish name that is not normally used by non-Jews is assumed to be Jewish (and vice-versa). Shu"t Zichron B'Sefer quotes from R' Moshe that the Russian Olim could be assumed to be Jewish if they spoke Yiddish and had Jewish on their Russian passport.