Gemara

A Nochri told a Yisrael 'Cut fodder and give it to my animals on Shabbos. If not, I will kill you like I killed Ploni the Yisrael, because he refused to cook for me on Shabbos.'

Sanhedrin 74a (R. Yochanan): Chachamim voted and concluded that if a person is told to transgress or else be killed, he may (or must) transgress, with three exceptions (for which he must forfeit his life) - idolatry, Arayos, and murder.

Shabbos 49a: Romi decreed to puncture the skull of anyone who will wear Tefilin. Elisha Ba'al Kenafayim wore Tefilin in the market. An officer appointed to enforce the decree saw him and chased him. When he caught him, Elisha took the Tefilin from his head and put it in his hand.

The officer asked what he is holding. Elisha said that it is dove's wings. A miracle was done, he opened his hand and revealed dove's wings.

Pesachim 53b (Beraisa - Tudus) Question: (Nebuchadnetzar made an image and decreed that everyone must bow to it or be thrown into a furnace.) Why did Chananyah, Misha'el and Azaryah enter the furnace to be Moser Nefesh?

Answer: They made a Kal va'Chomer from the Tzefarde'im. Even though frogs are not commanded about Kidush Hash-m, "U'Va'u b'Veisecha uv'Sanurecha uv'Mish'arosecha";

Kneading bowls are near ovens when the ovens are hot, yet the frogs entered. We are commanded about Kidush Hash-m, all the more so we should enter!

Rishonim

The Rif (Sanhedrin 17a) brings the Gemara in Sanhedrin.

Rambam (Hilchos Yesodei ha'Torah 5:4): If one refuses to transgress and is killed when he is exempt from Mesiras Nefesh, he is liable for his own death.

Milchamos Hash-m (Sanhedrin 17b): R. Tzadok and Rav Kahana were ready to kill themselves rather than sin with Nochriyos who intended for their own benefit (Kidushin 40a). Surely they would not go beyond the letter of the law to kill themselves! "V'Ach Es Dimchem l'Nafshoseichem Edrosh" forbids suicide. The Yerushalmi forbids Mesiras Nefesh beyond the letter of the law. We never find that it is praiseworthy for a sick person to refrain from transgressing to save his life.

Question: Elisha Ba'al Kenafayim merited that a miracle was done for him. Why wasn't he Moser Nefesh for Tefilin when the officer caught him? There was a decree against Tefilin!

Answer #1 (Ran Shabbos 22b DH u'Makshu): One must be Moser Nefesh only for a Lav. Alternatively, when the Nochrim can physically prevent him from doing the Mitzvah (e.g. they can imprison him), he is exempt from Mesiras Nefesh. It was Midas Chasidus for him to risk his life by wearing them. The same applies to Yisraelim who were Moser Nefesh for Milah and Lulav (Vayikra Rabah 32) and Tefilah (i.e. Daniel).

Question: The Rambam forbids Mesiras Nefesh when it is not required!

Answer #2 (Nimukei Yosef 18a DH veha'Mekadesh): If someone with great Yir'as Shomayim sees that the generation wantonly transgresses, he may be Moser Nefesh in order that people will learn from him to fear Hash-m. This is the answer (Pesachim 53b) to why Chananyah, Misha'el and Azaryah were Moser Nefesh. Really, the image was not idolatry; it was only for the king's honor. Nevertheless, people thought that it was idolatry, so Chananyah, Misha'el and Azaryah were voluntarily Moser Nefesh to inspire others.

Answer #3 (Kesef Mishneh): The Medrash connotes that they were obligated to be Moser Nefesh, and similarly regarding R. Chanina ben Tradyon and R. Yehudah ben Bava who were Moser Nefesh to learn Torah (Avodah Zarah 18a) and give Semichah (Sanhedrin 14a). Elisha removed the Tefilin because he was already Yotzei wearing them that day.

Rosh (Avodah Zarah 2:9): If one wants to be stringent and be Moser Nefesh for other Aveiros, he is permitted. This is not considered suicide. The Yerushalmi says that R. Aba was Moser Nefesh not to eat Neveilos. The episode was in a Nochri's house, presumably it was in private.

Talmid ha'Rashba (on Kil'ayim, Inyanim b'Sof ha'Sefer 7): One may allow himself to be killed for matters of Kidush Hash-m. Daniel was Moser Nefesh for Tefilah, and R. Chanina publicly taught Torah to multitudes with a Sefer Torah in his lap. R. Yosi ben Kisma did not say that this is forbidden; he merely advised against it lest the generation be left empty. This is why the Zekenim ordained by R. Yehudah ben Bava fled to save themselves (Sanhedrin 14a). Even though they were utter Chasidim, they were not Moser Nefesh lest fines be forgotten. This is why R. Meir did not correct the officer who misunderstood his answer (Avodah Zarah 16b). He would have been Moser Nefesh had there been others in the generation as great as himself. (We can answer similarly according to our text, which says that this episode involved R. Eliezer.) If one is charged for a matter that is not Kidush Hash-m, e.g. for theft or transgressing the king's law, he should escape if possible. If not, this is like suicide. R. Meir fled after freeing his wife's sister (Avodah Zarah 18a), even though he knew that miracles would be done to save even the officer, and all the more so himself if he wanted. This is because there is no Kidush Hash-m to be Moser Nefesh for such matters.

Poskim

Shulchan Aruch (YD 157:1): If a Nochri threatens a Yisrael to transgress a Mitzvah or be killed, regarding any Mitzvah other than idolatry, Arayos and murder, if it is in private, the Yisrael should transgress and not be killed. If the Nochri's intends to make the Yisrael transgress, the Yisrael may be stringent and submit to be killed.

Bach (DH u'Mah she'Chasav v'Im): SMaK rules like Tosfos and the Rosh, who permit Midas Chasidus to be Moser Nefesh when this is not required, like Sha'ul. However, perhaps this is only if one is concerned lest he succumb to the coercion to sin. The Medrash ("V'Ach Es Dimchem..." permits taking one's life (only) in a case like Sha'ul, i.e. one who fears lest he lest he succumb to the coercion to sin. Otherwise he is liable for his own death, like the Rambam says.

Question: Tosfos (Avodah Zarah 27b DH Yachol) permits Mesiras Nefesh when it is not required, but Tosfos (Sanhedrin 74b DH Ketol) says that Ploni (who was killed for refusing to cook for a Nochri on Shabbos - Yevamos 121b) erred!

Answer (Bach DH u'Mah she'Chasav ba'Meh): One may be Moser Nefesh only if the Nochri's intent is to make the Yisrael transgress, even if it is in public.

Shach (1,2): The Bach rules like the Rambam who says that anyone who should have transgressed to save his life and did not do so is liable for his own death. Normally, we are lenient regarding Safek Nefashos. However, regarding Kidush Hash-m the Torah was not concerned for loss of life, therefore this is unlike normal Sefekos.

Note: The Shach himself (7) says that we are lenient regarding Safek Nefashos regarding whether or not one must be Moser Nefesh at a time of Shemad if the Nochri intends for his own benefit.

Shach (ibid.): Some say that those who killed themselves out of concern lest they succumb to sin are Kedoshim, like Sha'ul; others argue and forbid. Even though the Beis Yosef rules like the Rambam, he allows Mesiras Nefesh when there is a need, i.e. the generation wantonly transgresses. One may rely on this.

Shulchan Aruch (ibid.): If it is in public, i.e. in front of 10 Yisraelim, if the Nochri's intent is to make the Yisrael transgress (Rema - even the Yisrael custom of shoelaces), the Yisrael must be Moser Nefesh. If the Nochri intends for his own benefit, the Yisrael should transgress and not be killed.

Rema: This is only if they want Yisraelim to transgress a Lav. One need not be Moser Nefesh for an Aseh. However, if it is necessary in this era and the Yisrael wants to fulfill it and be killed, he may.

Beis Yosef (DH v'Kosvu Od): Bereishis Rabah (34:14) forbids suicide from "V'Ach Es Dimchem l'Nafshoseichem Edrosh" (our text learns from "Ach Basar..."). It suggests that this applies even to Chananyah, Misha'el and Azaryah, i.e. at a time of Shemad. "Ach" refutes this. The Medrash suggests that this applies even to Sha'ul, i.e. one who fears that he will be tortured greatly. "Ach" refutes this. Some use this to justify slaughtering children at a time of Shemad when there is a royal decree. Others forbid. They explain "Ach" to forbid taking one's own life. Chananyah, Misha'el and Azaryah did nothing to harm themselves, they only put themselves in the hands of others. This opinion holds that Sha'ul acted improperly.

Rebuttal (Shirei Keneses ha'Gedolah, brought in Hagahos to Tur ha'Shalem 2): Surely, Sha'ul acted properly! His place in the world to come is with Shmuel ha'Navi. After his death a Bas Kol announced that he is "Bechir Hash-m" (Berachos 12b). Rather, he was permitted because he knew that he would not live (Shmuel told him that he would die that day) and he feared lest the Plishtim make a mockery of him. If it is possible that one will live, he may not strike himself.

Beis Yosef (ibid.): A case occurred, at a time of Shemad a Rav slaughtered children lest they be forced to transgress. Another Rav called him a murderer, and said 'If I am right that Rav should die a cruel death.' This was fulfilled, and the decree ceased; perhaps the children would have survived had they not been slaughtered.