Gemara

61b (Abaye): If one served idolatry due to love or fear, he is liable, for in any event he served.

(Rava): He is exempt, for he did not accept it to be his god.

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

93a (Beraisa): When Nebuchadnetzar was forcing people to bow to the image or be burned, he wanted Daniel to leave, lest people say that he burned his own god (he worshipped Daniel). Hash-m wanted Daniel to leave, lest people think that Chananyah, Misha'el and Azaryah were saved in Daniel'is merit. Daniel wanted to go, lest he be burned (idolatry should be burned).

Kesuvos 3b (Beraisa): From the time of danger and onwards, the custom was to be married on Tuesday. Chachamim did not object. One may not be married on Monday. If there is an Ones, it is allowed.

(Rava): The danger was a decree that a virgin married on Wednesday must first have Bi'ah with the mayor. It was Ones, and not just a danger, for virtuous women would resist, and be killed for doing so.

Question: We should publicize that if a woman is forced, she is permitted to her husband!

19a (Rav Chisda): R. Meir holds that a witness should forfeit his life rather than sign falsely, so he is not believed to say that he was forced to sign.

Objection (Rava): If he would ask, we would tell him to sign and live. Why shouldn't we believe him?!

Pesachim 25a (Ravin citing R. Yochanan): One may heal through any means, except through idolatry, Arayos or murder.

(Beraisa - R. Eliezer): "Uv'Chol Nafshecha" obligates loving Hash-m with all his life (he may not transgress idolatry, even to save his life).

25b: Reasoning teaches about murder. You do not know whether you are more worthy to live than another person!

(Beraisa - Rebbi): "Ki ka'Asher... u'Rtzacho Nafesh" equates Na'arah Me'orasah to a murderer. Just like one may not murder to save his life, he may not transgress Arayos.

53b (Beraisa - Tudus): Why did Chananyah, Misha'el and Azaryah enter the furnace? They made a Kal va'Chomer. The frogs are not commanded about Kidush Hash-m, yet they entered ovens. All the more so we should enter!

Rishonim

Rambam (Hilchos Yesodei ha'Torah 5:4): If Mesiras Nefesh was required and one transgressed, he was Mechalel Hash-m. If 10 Yisraelim were present, the Chilul Hash-m was b'Rabim. However, we lash or kill only one who transgressed willingly with warning. It says about idolatry "ha'Hu" will get Kares, but not one who was Ones. All the more so he is not killed, and all the more so for other Aveiros. Regarding Arayos it says "vela'Na'arah Lo Sa'aseh Davar."

Rambam (6): Sickness is like Ones. If one was mortally sick, he may be cured through any Isur, except for idolatry, Arayos or murder. If he violated this and was cured through them, Beis Din gives the appropriate punishment.

Minchas Chinuch (295 DH ul'Fi):Also Sefer ha'Chinuch says that Beis Din does not punish for Chilul Hash-m due to Anus, but they punish a Choleh who transgressed one of the three. What is the distinction? A Choleh is like one who was pursued. If he broke Kelim, he must pay for them. If one was coerced to damage another's money, he is exempt. Healing oneself through Isur is not Ones. Rather, he saves himself through Isur.

Kovetz Shi'urim (Kesuvos 8): When he was forced to do the act, it is not considered his action. No one forced the Choleh to transgress, therefore, it is considered that he transgressed idolatry.

Teshuvas ha'Rosh (54:1): If one was Hemir (became an idolater) due to mortal coersion, and immediately repented when the coersion ceased, he is Kosher for testimony. Even if he could have fled the coercion, and he did not, and he ate Neveilah for pleasure, he is disqualified for that time, but not for when he had no escape. If he was Kosher when he saw testimony, and also now, even if he was Pasul in between, he may testify, like a stranger who married the litigant's daughter, and became a stranger again. However, one must check. Many who were Hemir at a time of decrees were suspected even before the decree.

Rivash (4): If one transgressed even idolatry and was not Moser Nefesh, he is like a Yisrael in every way. He is Kosher for testimony. Beis Din does not punish him, for the Torah exempted him, even if it was in front of 10 and he was Mechalel Hash-m. At the time of decrees, if an Anus transgresses even in private, he is Pasul. Many Anusim, rich and poor, escaped the coercion. We must investigate one who did not, to see if he keeps Mitzvos in private.

Tosfos (Avodah Zarah 54a DH Ha): "Va'Chay Bahem" exempts from all punishment for the three Aveiros. "Uv'Chol Nafshecha" obligates Mesiras Nefesh even in a case of Ones, and "V'Lo Sechalelu" teaches that the warning and Chiyuv Misah apply.

Ri (in Tosfos Pesachim 53b DH Mah): Chananyah, Misha'el and Azaryah could have fled, like Daniel. The Gemara asked why they did not flee.

Yabi'a Omer (6 YD 13:6): This was considered in public. Do not say that since the others bowed, they are Mumarim and like Nochrim. They bowed due to mortal Ones, so they are like Yisre'elim. The Maharsha said that only Daniel was able to flee, for Nebuchadnetzar wanted him to. Shir ha'Shirim Rabah (7:13) connotes that Chananyah, Misha'el and Azaryah could have fled. Yechezkeil counseled them to wait for the storm to pass. The Rivash's (171) text of the Rambam (Yesodei ha'Torah 5:4) says 'if one can flee from the evil king but he does not, he is like one who served b'Mezid. He loses Olam ha'Ba and goes to the lowest level in Gehinom.' Megilah 12a said that Yisrael were liable for bowing to the image, for they could have fled.

Poskim

Shulchan Aruch (YD 119:9): If a convert reverted to idolatry due to fear, or if a Yisrael sinned due to fear lest he be killed, he is a full Yisrael. What he slaughters is permitted, and he does not forbid wine that he touches.

Gra (23): The Halachah follows Rava, who exempts one who served idolatry due to fear, even from lashes. The Rambam learns from "vela'Na'arah Lo Sa'aseh Davar." We learn also from Kesuvos 3b. "Ha'Hu" exempts Ones from Kares (Toras Kohanim). Since Beis Din does not punish him, he may testify. The Rema disqualifies mid'Rabanan, but he is believed about Isurim, for he is a full Yisrael. The Torah believes even a thief about Isurim; we disqualify him mid'Rabanan only after announcing about him.

Rema (157:1): If one was obligated to be Moser Nefesh, but transgressed, he was Mechalel Hash-m. Nevertheless, he is exempt.

Gra (18): This is like the first answer in Tosfos (61b). The second answer says that (in a usual case) one who transgressed is liable.

Rema (ibid.): However, if he could have fled, he is considered Mezid.

Shulchan Aruch (CM 34:2): A Rasha is Pasul for testimony if he transgressed an Aveirah for which one is lashed, and all the more so if he is Chayav Misas Beis Din, whether he transgressed for lust or to anger Hash-m.

Rema: If one transgressed an Aveirah without lashes, he is Pasul mid'Rabanan.

Shach (35:7): Maharan Sason (90) disqualifies testimony of Anusim even after they resume to be Kosher, for the beginning and end must be in Kashrus. This is wrong. An Anus was Kosher; he was a Yisrael between himself and Shamayim (when the Nochrim did not see).

Shulchan Aruch (EH 17:6): At a time of decrees, we permit a widow through witnesses (of the death) who were Hemiru due to Ones and repented.

Noda b'Yehudah (1 EH 74): We hold that one who was not Moser Nefesh is Kosher for testimony. Why does R. Meir disqualify one who signed b'Ones? The Sho'el compared this to a man who transgressed Arayos. This is not called Ones, for erection only comes willingly. Here also, they signed willingly! This is wrong. They were coerced to sign! Rather, "va'Chay Bahem" applies to the entire Torah. There is a Limud that for the three Aveiros, one must be Moser Nefesh. If he did not, he transgressed only this Limud, and not the Aveirah itself. There is no Lav, he is not lashed and he is not called a Rasha. If R. Meir holds that "va'Chay Bahem" does not apply to false testimony, there is a Lav and he becomes a Rasha.

Note: It is not clear to me why we don't say that the Limud shows that "va'Chay Bahem" does not apply to the three Aveiros!

Ketzos ha'Choshen (35:4 DH v'Ayen): Even though one who transgressed b'Ones is not disqualified from testimony, he is a Rasha, so R. Meir holds that one is not believed to say that he siged due to Ones.

Shev Shematsa (7:5): Granted, the Rambam says that one who sinned b'Ones transgressed only v'Nikdashti. He is not a Rasha, for this is not a Lav with lashes or death. However, he should be Pasul for testimony! One who transgressed an Aseh is Kosher if he did so to anger Hash-m; we are not concerned lest he did so due to lust for money. We must say that here his Yetzer (to save his life) overcame him. One who transgressed Ervah is Pasul, for he could have overcome his Yetzer. The Rashba says that R. Meir holds that it is Midas Chasidus to be Moser Nefesh and refuse to sign Sheker, and still one is not believed to say that he was not Moser Nefesh!

Question (Kovetz Ha'aros 51:2): Surely the initial Lav still applies. If one transgresses only the Aseh v'Ohavta, we would not need a verse to exempt from Misah! Also, we should lash for transgressing Chilul Hash-m, which applies even amdist Ones! Tosfos proved that one who was Mechalel Hash-m transgressed idolatry. Why does the Rivash Machshir him for testimony? Beis Din does not punish him, but he transgressed a Lav with lashes! This should be like one who transgressed b'Mezid without warning!

Answer (Kovetz Shi'urim Kesuvos 6,7): We need the verse to exempt only there was a Chiyuv to be Moser Nefesh, but not if she was passive. We must say when one does an action due to Ones, it is not considered his action. Therefore, he is Kosher for testimony.