Shavuos III - Whoever rejects idolatry is called a Jew

Being a Jew is not a just a racial, biological or identity issue. It also involves beliefs. Perhaps the most important one is the rejection of idolatry and the acceptance of Torah.
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Megila(13a): R’ Yochanon said that Mordechai did in fact come from the tribel of Binyamin. So why was he called “a Jew” [which implies that he was from the tribe of Yehudah]? Because he rejected idolatry since anyone who rejects idolatry is called a Jew [even though he isn’t from the tribe of Yehudah] as we see in Daniel (3:12):“There are certain Jews whom you have set over the affairs of the province of Babylon, Shadrach, Meshach, and Abed-Nego; these men, O king, have not regarded you; they serve not your gods, nor worship the golden image which you have set up.”

Meiri(Megila 13a): Whoever rejects idolatry is as if he accepted the entire Torah. Whoever raises an orphan in his house is as if he gave birth to the orphan.

Mahari Bruno((#135): There was an incident with a person who took an oath not to play with any Jew. R’ Isserlin was asked whether he could play with an apostate (mushuad)? He was allowed to since an apostate is not called a Jew. Even though Sanhedrin (44a) states that a sinner is still consdiered to be Yisroel – nevertheless he is not called a Jew and therefore the oath doesn’t apply. A proof for this is that Rashi (Sanhedrin 44a) explains the term Jew based on the Megila (13a) that whoever rejects idolatry is called a Jew. Therefore an apostate who rejects the G‑d of Israel and accepts idolatry is not called a Jew.

Rabbeinu Bachye(Devarim 8:1): Our Sages (Shavuos 29a) teach that whoever rejects idolatry is as if he acknowledges the entire Torah and whoever acknowledges idolatry is as if he rejected the entire Torah.

R' Ezriel Hildsheimer(Y.D. 1:259): Question: Is a person called a Jew based upon Megila (13a) that whoever rejects idolatry is called a Jew? Answer: This is one of the statements which causes great destruction amongst us. It is obviously not so. Heaven forfend that one should understand this statement literally...

Yad Ramah (Sanhedrin 19b): Calev ben Yefunah’s wife was called a Jewess even though she was the daughter of Pharaoh – but since she rejected idolatroy she was called a Jewess. This principle is learned from Daniel (3:12): “There are certain Jews [members of the tribe of Yehuda – Rashi- whom you have set over the affairs of the province of Babylon, Shadrach, Meshach, and Abed-Nego; these men, O king, have not regarded you; they serve not your gods, nor worship the golden image which you have set up.” This verse indicates that whoever rejects idolatry is called a Jew.

Rambam(Hilchos Avodas Kokovim 2:4): The commandment prohibiting idolatry is as significant as all the other mitzvos. This is learned from Bamidbar (15:22): And if you have erred, and not observed all these commandments, which the L‑rd has spoken to Moses, We have a tradition that this verse is talking about idolatry. Therefore we learn from this verse that whoever agrees with idolatry denies the entire Torah as well as all the prophets and all that is written in the prophets from Adam until the end of the world (Bamidbar 15:23). Similarly one who rejects idolatry agrees with the entire Torah and all the prophets and all that which was commanded by the prophets from Adam until the end of the world. Thus it is the foundation of all the mitzvos.