Being Nosy About Someone's Conversion is Forbidden by Jewish Law

לֹֽא־תִקֹּ֤ם וְלֹֽא־תִטֹּר֙ אֶת־בְּנֵ֣י עַמֶּ֔ךָ וְאָֽהַבְתָּ֥ לְרֵעֲךָ֖ כָּמ֑וֹךָ אֲנִ֖י יְהֹוָֽה׃

You shall not take vengeance or bear a grudge against your countrymen. Love your fellow as yourself: I am the LORD.

Someone who converted is your "countryman." Love this person as you would love yourself and your loved ones, meaning, don't ask them invasive and personal questions that will make them feel alienated, exposed, and different. Welcome them with warmth and help them feel like they belong, just as you would want someone to help you feel like you belong. That is what"V'ahavta l'rei'acha kamocha" is all about.

בבא מציעא נ״ח ב:י״ב-י״ג

תני תנא קמיה דרב נחמן בר יצחק כל המלבין פני חבירו ברבים כאילו שופך דמים א"ל שפיר קא אמרת דחזינא ליה דאזיל סומקא ואתי חוורא אמר ליה אביי לרב דימי במערבא במאי זהירי א"ל באחוורי אפי דאמר רבי חנינא הכל יורדין לגיהנם חוץ משלשה הכל ס"ד אלא אימא כל היורדין לגיהנם עולים חוץ משלשה שיורדין ואין עולין ואלו הן הבא על אשת איש והמלבין פני חבירו ברבים והמכנה שם רע לחבירו מכנה היינו מלבין אע"ג דדש ביה בשמיה

Bava Metzia 58b:12-13

The Gemara relates that the tanna who recited mishnayot and baraitot in the study hall taught a baraita before Rav Naḥman bar Yitzḥak: Anyone who humiliates another in public, it is as though he were spilling blood. Rav Naḥman bar Yitzḥak said to him: You have spoken well, as we see that after the humiliated person blushes, the red leaves his face and pallor comes in its place, which is tantamount to spilling his blood. Abaye said to Rav Dimi: In the West, i.e., Eretz Yisrael, with regard to what mitzva are they particularly vigilant? Rav Dimi said to him: They are vigilant in refraining from humiliating others, as Rabbi Ḥanina says: Everyone descends to Gehenna except for three. The Gemara asks: Does it enter your mind that everyone descends to Gehenna? Rather, say: Anyone who descends to Gehenna ultimately ascends, except for three who descend and do not ascend, and these are they: One who engages in intercourse with a married woman, as this transgression is a serious offense against both God and a person; and one who humiliates another in public; and one who calls another a derogatory name. The Gemara asks with regard to one who calls another a derogatory name: That is identical to one who shames him; why are they listed separately? The Gemara answers: Although the victim grew accustomed to being called that name in place of his name, and he is no longer humiliated by being called that name, since the intent was to insult him, the perpetrator’s punishment is severe.

When you ask someone about their Jewish legitimacy, you are humiliating them. Imagine someone who moved to New York City 10 years ago, and having someone say to them "You're from New York? I don't know about that -- you don't seem like a New Yorker. Where exactly did you grow up? What high school did you go to?" This feeling of making someone feel insecure and defensive, the Gemara describes this pain to be so harmful, it is as if you had spilled their blood.

The Gemara says that we will all descend into Gehenna at the end of our lives, but we will eventually become purified and ascend out of it and go to olam haba. However, there are a few folks who won't ascend. These are folks who have humiliated someone in public and those who have called someone a derogatory name. This would include telling someone "You aren't a real Jew because you converted Reform." That is humiliating. You're telling them: "You're a goy" or "You're a shiksa" -- which are "derogatory names."

בבא מציעא נ״ט א:ב׳

ולא עוד אלא אפילו בשעה שעוסקין בנגעים ואהלות אומרים לי דוד הבא על אשת איש מיתתו במה ואני אומר להם מיתתו בחנק ויש לו חלק לעוה"ב אבל המלבין את פני חבירו ברבים אין לו חלק לעוה"ב

Bava Metzia 59a:2

And moreover, they torment me to the extent that even at the time when they are engaged in the public study of the halakhot of leprous sores and tents in which there is a corpse, i.e., halakhic matters that have no connection to my sin, they say to me: David, one who engages in intercourse with a married woman, his death is effected with what form of execution? And I say to them: One who engages in intercourse with a married woman before witnesses and with forewarning, his death is by strangulation, but he still has a share in the World-to-Come. But one who humiliates another in public has no share in the World-to-Come. The transgression of you, who humiliate me, is more severe than my transgression.

If you humiliate someone in public, you will not be deserving of going to olam haba. Unrelated to conversion, but still worth noting: humiliating someone for committing a sin is even more serious than the sin itself. Your invasive questions can make someone feel really uncomfortable.

בבא מציעא נ״ט א:ג׳

(ואמר) מר זוטרא בר טוביה אמר רב ואמרי לה אמר רב חנא בר ביזנא אמר ר"ש חסידא ואמרי לה א"ר יוחנן משום רשב"י נוח לו לאדם שיפיל עצמו לכבשן האש ואל ילבין פני חבירו ברבים מנ"ל מתמר דכתיב (בראשית לח, כה) היא מוצאת והיא שלחה אל חמיה

Bava Metzia 59a:3

And Mar Zutra bar Toviyya says that Rav says; and some say Rav Ḥana bar Bizna says that Rabbi Shimon Ḥasida says; and some say Rabbi Yoḥanan says in the name of Rabbi Shimon ben Yoḥai: It is more comfortable for a person to cast himself into a fiery furnace, than to humiliate another in public to avoid being cast into the furnace. From where do we derive this? From Tamar, daughter-in-law of Judah. When she was taken out to be burned, she did not reveal that she was pregnant with Judah’s child. Rather, she left the decision to him, to avoid humiliating him in public, as it is written: “And Judah said: Bring her forth, and let her be burnt. When she was brought forth, she sent to her father-in-law, saying: I am pregnant by the man to whom these belong. And she said: Examine these, whose are these, the signet, and the cords, and the staff?” (Genesis 38:24–25).

Tamar took the law of not humiliating someone so seriously, she was willing to be burned alive, rather than save herself by exposing her pregnancy which would humiliate Judah. (This story is very problematic.)

בבא מציעא נ״ט ב:י״ב

תנו רבנן המאנה את הגר עובר בשלשה לאוין והלוחצו עובר בשנים

Bava Metzia 59b:12

§ The Sages taught: One who verbally mistreats the convert violates three prohibitions, and one who oppresses him in other ways violates two.

Verbally mistreating someone who converted (asking them personal questions and inquiring about their legitimacy) violates three laws, and oppressing them violates two laws. Unless you are that person's Cantor or Rabbi, it is not your job (and none of your business) to tell them about the difficulties of being married and having children in Israel. It can feel very oppressive.

בבא מציעא נ״ט ב:י״ג

מאי שנא מאנה דכתיבי שלשה לאוין (שמות כב, כ) וגר לא תונה (ויקרא יט, לג) וכי יגור אתך גר בארצכם לא תונו אותו (ויקרא כה, יז) ולא תונו איש את עמיתו וגר בכלל עמיתו הוא לוחצו נמי שלשה כתיבי (שמות כב, כ) ולא תלחצנו (שמות כג, ט) וגר לא תלחץ (שמות כב, כד) ולא תהיה לו כנושה וגר בכלל הוא אלא אחד זה ואחד זה בשלשה

Bava Metzia 59b:13

The Gemara asks: What is different with regard to verbal mistreatment, that three prohibitions are written concerning it: “And you shall neither mistreat a convert” (Exodus 22:20); “And when a convert lives in your land, you shall not mistreat him” (Leviticus 19:33); “And you shall not mistreat, each man his colleague” (Leviticus 25:17), and a convert is included in the category of colleague? With regard to one who also oppresses a convert as well, three prohibitions are written: “And you shall neither mistreat a convert, nor oppress him” (Exodus 22:20); “And you shall not oppress a convert (Exodus 23:9); “And you shall not be to him like a creditor” (Exodus 22:24). This last prohibition is a general prohibition, in which converts are included. Consequently, it is not correct that one who oppresses a convert violates only two prohibitions. Rather, both this one, who verbally mistreats a convert, and that one, who oppresses him, violate three prohibitions.

This section highlights many passages in Torah that command us to treat those who have converted with respect. It refers to someone who has converted to be a colleague. You would not go to a colleague and act as if you are a "creditor," interrogating them and demanding answers.

בבא מציעא נ״ט ב:י״ד

תניא רבי אליעזר הגדול אומר מפני מה הזהירה תורה בל"ו מקומות ואמרי לה במ"ו מקומות בגר מפני שסורו רע

Bava Metzia 59b:14

It is taught in a baraita that Rabbi Eliezer the Great says: For what reason did the Torah issue warnings in thirty-six places, and some say in forty-six places, with regard to causing any distress to a convert? It is due to the fact that a convert’s inclination is evil, i.e., he is prone to return to his previous way of living.

If you cause a Jew-by-choice to feel uncomfortable and stressed about being in Jewish spaces, you will cause them to leave. They may very well return to their previous religion, which would be such a shonde! One thing the world doesn't need less of is another Jew.

בבא מציעא נ״ט ב:ט״ו

מאי דכתיב וגר לא תונה ולא תלחצנו כי גרים הייתם בארץ מצרים (תנינא) רבי נתן אומר מום שבך אל תאמר לחברך והיינו דאמרי אינשי דזקיף ליה זקיפא בדיותקיה לא נימא ליה לחבריה זקיף ביניתא:

Bava Metzia 59b:15

What is the meaning of that which is written: “And you shall not mistreat a convert nor oppress him, because you were strangers in the land of Egypt” (Exodus 22:20)? We learned in a baraita that Rabbi Natan says: A defect that is in you, do not mention it in another. Since the Jewish people were themselves strangers, they are not in a position to demean a convert because he is a stranger in their midst. And this explains the adage that people say: One who has a person hanged in his family [bidyotkei], does not say to another member of his household: Hang a fish for me, as the mention of hanging is demeaning for that family.

This section really speaks for itself. Empathy! Relating to someone! We were strangers in the land of Egypt. We were nervous, afraid, and in need of kindness and understanding. So when you ask inappropriate questions -- questioning a Jew's belonging in a Jewish space -- you demean them and forget your own journey in Mitsrayim.