Gemara

(Rav and Shmuel): Even a Tinok she'Nishba (a baby captured) by Nochrim or one who converted among Nochrim is like one who knew about Shabbos and forgot. (He brings a Korban for Chilul Shabbos.)

(R. Yochanan and Reish Lakish): One who knew about Shabbos and forgot is liable, but a baby captured by Nochrim or one who converted among Nochrim is exempt.

Rishonim

Rambam (Hilchos Mamrim 3:3): Anyone may kill one who is Kofer in (i.e. denies) oral Torah based on his thinking, and follows his flimsy opinion and freedom of heart, like Tzadok, Baisus and all those who strayed after them. The children and grandchildren of those who strayed, who were corrupted by their fathers and grew up among Kara'im (another group that did not accept oral Torah) and were raised with their creed, are like Tinokos she'Nishbe'u among Nochrim, for they are like Anusim (forced). Even if afterwards one heard that he is a Yisrael, and he saw Yisre'elim and their laws, he is like Anus, for he was raised with their mistake. It is proper to bring them to repent, and draw them with words of Shalom until they return to Torah.

Radvaz: The Rambam wrote this to defend the Kara'ites. Those found nowadays, if we could be Morid them (make them fall into a pit and die), this would be a Mitzvah, for every day we [try to] get them to repent and draw them to believe in oral Torah, and they revile and blaspheme those who have the tradition. We should not judge them like Anus, rather, like Kofrim to oral Torah.

Radvaz (Teshuvah 2:796, printed at the end of volume 2): The Rishonim said 'we are Morid [heretics], and do not raise them [from a pit], and one who transgresses Chachamim's words is Chayav Misah.' It is agonizing to hear those who want to be Machshir testimony, Gitin, bread and wine of these Kara'ites, and to intermarry with them. R. Shimshon was very stringent about them. An Acharon negated his words and permitted everything that Chachamim were stringent about. We should not justify those who deny the tradition.

Radvaz: A Beraisa equates Chilul Mo'ed with Chilul Shabbos. The Kara'ites are Mechalel Mo'ed, so they are like Mechalelei Shabbos who deny the entire Torah, so their wine is like that of Nochrim. Giving one's daughter to one who denies oral Torah is like expelling her [from Torah observance]. Women are easily enticed. Also, she will constantly transgress "v'Lifnei Iver Lo Siten Michshol", since she will cook her husband's forbidden food for him. Tosfos says that one may not give to a Mumar the Mumar's forbidden food, for he will surely eat it and he is considered a Yisrael.

Suggestion: One may marry a Kara'ite girl, for a man rules in his house, and we are not concerned lest he be enticed. He can feed her food permitted to us and to them, e.g. a Tzeduki slaughtered and a Yisrael supervised. He can set a Yisrael to supervise salting and removal of Chelev, Gid ha'Nasheh...

Rejection (Radvaz): This is totally wrong. They are not concerned for "v'Lifnei Iver." The supervisor cannot ensure that she will not let a Nochri touch the wine, or put a dairy spoon in a meat pot, or Chelev in the pot to improve her portion. One may not intermarry with them, for a Kofer to oral Torah is Chayav Misah b'Yedei Adam. The Rambam said that they are Anusim. He said that we are not quick to kill them. This implies that if we warned them and they still rebel, we kill them if we can.

Radvaz: Indeed, the Rambam (Hilchos Eruvin 2:16) says that we do not make an Eruv with Tzedukim, Baisusim and those who deny oral Torah, for they do not admit to an Eruv. We cannot bring a proof from Eruvin, which is mid'Rabanan. The Rambam is Machshir if a Tzeduki slaughtered in front of us. I say that he discusses a Tzeduki who errs (follows what he was told), but not one who b'Mezid denies Chachamim's words. He is Machshir a Get with a Tzeduki witness, for they are unlike Kusim (Samaritans, descendants of the Nochrim that Sancheriv settled in place of the 10 tribes after he exiled them. Their conversion was dubious, and they did not observe all the Mitzvos.) This refers to one who follows his fathers' customs, but he accepts when we tell him Torah law.

Chazon Ish (YD 2:18, Hagahah): The Rambam says that we kill Kofrim. One is exiled for killing a Kusi (Makos 8b)! That refers to their children, who are considered Shogeg. Their Shechitah is Kosher when Yisre'elim supervise.

Radvaz: Nowadays, Kara'im are Mechalel Shabbos in everything mid'Rabanan, and perhaps even in Torah Melachos, for no one explained the laws to them. One may teach written Torah to them, but not oral Torah, unless they accept to repent. If not, they will mock Chachamim's words. They learn only in order to argue! I say that there is no concern for Mamzerus among them, for they do not have Kidushin in front of Kosher witnesses. If one repents, I permit him to marry into Yisrael.

Mizrachi (57): Kara'im nowadays descend from those who chose to deny almost 2000 years ago. They are like Tinokos she'Nishbe'u. Even if one heard afterwards that he is a Yisre'el, we need not punish him, for he is like Anus. The Rambam says so, and that we should not rush to kill them. Do not infer that if we warned them and they still rebel, we should kill them. He already compared them to a Tinok she'Nishba, who is not liable at all! Even if he did not want to repent, he is like Anus.

Nimukei Yosef (Bava Metzi'a 42b DH v'Kivan): We hold (Avodah Zarah 26b) that anyone whom we are Morid is not considered Achicha (your brother, so you may lend to him with usury). A Tinok she'Nishba is considered Achicha, for he never saw Torah law. A Mumar's child lives among Yisre'elim, yet acts like a Nochri. We are Morid him.

Rashbash (89): The Rambam says that a denier's child is like a Tinok she'Nishba. Perhaps he discusses one who saw Yisre'elim and their creed, but no one made him understand the truth of their creed. If he understands that it is true and does not return to it, surely Moridin. There is no Ones! The Rambam discusses one 'corrupted by his fathers.' He trusts them and assumes that they would not want him to believe in Sheker.

Suggestion (in Mabit 1:37): We can permit Tzedukim to marry Yisre'elim. If the witnesses on their Gitin are Pesulim, also their Kidushin is invalid. Really, their witnesses are valid. One is disqualified only for transgressing b'Mezid. Tzedukim are like Anusim. They act like their fathers.

Rebuttal (Mabit): Being Shogeg in one matter, e.g. Lo Tachmod, is unlike Shogeg in many Kerisos that apply every day. If Tzedukim were Kosher mid'Oraisa, they would be Kosher even mid'Rabanan, like extortionists who pay for what they take, and think that the Torah permits this. Further, the Tzedukim, and even their descendants, are Mezid. They know that they descend from Yisrael, and they deny oral Torah, distort verses and are brazen to Chachamim. This is the ultimate Mezid! The Rambam calls them Tinokos she'Nishbe'u regarding the Mitzvah to try to get them to repent, and not to rush to kill them, but they are Pasul for testimony. Chachamim did not need to mention this, for they already taught "heretics". The Rambam is Machshir a Tzeduki on a Get only if he is a Chaver who acts like zealously observant Yisre'elim. Kusim were Pasul for testimony only mid'Rabanan. They observed Torah laws, so they would not testify falsely. They never accepted oral Torah. Tzedukim came from Bnei Yisrael, and they transgress Kerisos not explicit in the Torah. They are like Mezid.

Poskim

Shulchan Aruch (YD 159:3): One may lend to a Mumar with Ribis, but one may not borrow from him with Ribis.

Beis Yosef (DH ul'Inyan): The Rambam says that Kara'im are like Tinokos she'Nishbe'u, so one may not charge them Ribis. The Nimukei Yosef says that perhaps they are like Mumarim, for they are among Yisrael and choose to act like Nochrim. We do not abandon the Rambam's explicit opinion due to the Nimukei Yosef's Safek.

Beis Yosef (OC 385 DH u'Mah she'Chasav Afilu): Tosfos (69a DH Kan) explained that Tzedukim can be Mevatel Reshus because they keep even mid'Rabanan laws of Shabbos, for they fear Chachamim. Nowadays, the Tzedukim transgress mid'Rabanan laws of Shabbos, so they cannot be Mevatel. The Rambam would not say that Tzedukim nowadays are like Mumarim, for they are not Mezid. They are like Ohnes; they merely follow their fathers' ways. This is like a baby captured by Nochrim, like he wrote in Perush ha'Mishnayos (Chulin 1:1).

Chazon Ish (YD 1:6): The Shechitah of a Tinok she'Nishba is Kosher. The Chazakah is that if they would inform him and try the proper amount to return him in Teshuvah, he would repent. If they tried and he is still Mezid, he is a Mumar. The judges use their Ru'ach ha'Kodesh to decide what is the proper amount. The Acharonim argue about Tzedukim nowadays regarding what is the proper amount of trying.

Chazon Ish (2:28): One may not hate a Rasha until after rebuking him (Hagahos Maimoniyos Hilchos De'os 6:1). Ahavas Chesed (at the end) says that we must love Resha'im, for this is like before they were rebuked, for we do not know how to rebuke.

Igros Moshe (OC 4:91:6): One should not make a Minyan in a conservative Beis ha'Keneses, even in another room. They deny many Torah laws. Even one who denies one law is called a Kofer, and he is like a Min. Even if they are Shogeg, like Tinokos she'Nishbe'u, since their fathers and environment misled them, so we are not Morid them, in any case they are Kofrim and one must distance from them. The Rambam says to strive to draw them close to repent. This does not apply to conservatives in their Beis ha'Keneses, and not all are qualified to do so. In an orthodox Beis ha'Keneses without a proper Mechitzah or with a microphone, the congregants do not deny Mitzvos, just they belittle them. They believe in all Torah Mitzvos, just after transgressing many times these matters became like Heter to them. Intrinsically they are Kosher Yisre'elim. Sometimes they are totally Shogeg. There is no reason to distance from them. One may make a Minyan in the Beis ha'Keneses, but in another room, lest he be suspected of praying there. It is permitted after it is known that there is another Minyan (with a Mechitzah, without a microphone), or even before, if there is a separate entrance.

Shevet ha'Levi (9:198): The Rambam (Hilchos Shegagos 2:6) says that one is a Tinok she'Nishba only until he knows that he is a Yisrael. Even if he grew up among Nochrim, if now he lives among Yisrael and knows that he is a Yisrael, even if sometimes we cannot consider him Shogeg, he is no worse than a Mumar who transgresses due to lust, but not to anger Hash-m. He is Anus, but it is hard to consider him a Tinok she'Nishba.

Shuvah Alai (p.17): It is said in the name of ha'Gaon R. Y.S. Elyashiv Ztz"l that in Eretz Yisrael we cannot consider one to be a Tinok she'Nishba, for surely he heard of Yisre'elim who observe Mitzvos. However, one should consider him a Tinok she'Nishba to be stringent, e.g. for Hashavas Aveidah and Kivud Av v'Em.