Some years back I attended a talk by Rabbi Dr. M.D. Tendler who spoke about the most important teshuvot written by his father-in-law, Rav Moshe Feinstein zt”l. Amongst the most important teshuvot he discussed, was his allowance of observant parents whose children had become irreligious to eat at their children’s house (see Yoreh Deah #1:54). One could, Rav Moshe argued, rely on the assurances of the children that out of respect to their parents they would serve them only kosher food. How tragic it would be if parents and children could not eat together. .
“Rava said: In the case of a mumar, an Israelite apostate who eats carrion, letaiavon, in order to satisfy his lust, one prepares the knife and gives it to him, and then we may eat of his slaughtering’. What is the reason for this? Since there is the possibility to slaughter the animal in a permitted manner or to slaughter the animal in a prohibited manner, one does not intentionally forsake the permitted manner and eat food slaughtered in a prohibited manner” (Chulin 4a).
Rava argues that one who eats non-kosher food out of “lust”—because it is cheaper or perhaps he just enjoys non-kosher food better—all things being equal they would be happy to eat kosher food. While they will go to little effort to do so, if we provide the knife there is no reason to believe they will purposely slaughter it improperly. If the person refuses to eat kosher, lehachis, out of spite—kashrut in their mind reflecting an outdated system they want nothing to do with—then even giving them a ready-made knife would matter little. We must fear that they would purposely invalidate the shechita and hence we may not eat from it.
The Gemara attempts to prove that Rava is correct from the teaching that “All may shecht, even a Kuti, even one who is uncircumcised and one who is a mumar, an apostate” (Chulin 4b). There is little reason for the Sages to teach that one whose brothers died from circumcision (thus forbidding one to circumcise the younger brothers) is allowed to shecht, that is obvious; rather, we must be discussing one who rejects the notion of circumcision i.e. they are mumar learalot, and yet they still may shecht. Once we know that one who rejects a basic mitzva of the Torah such as circumcision may still shecht, the reference to a mumar can refer only to one who actually does not keep kosher. Thus our Sages teach that we can eat from the shechita even of one who does not keep kosher—thereby proving that Rava’s claim is correct.
The Gemara rejects this proof arguing that Rava is not correct and thus a mumar letaiavon may not shecht. Since he is so used to eating non-kosher food, kosher and non-kosher become indistinguishable. He may have no desire to serve others non-kosher food but the notion of kashrut is so far removed from his consciousness that he does not even realize how careful he must be to prepare kosher food. Sadly, this describes many contemporary Jews who are aware that religious Jews keep kosher, and they may even keep a modicum of kashrut themselves, but really have no understanding of what kashrut is all about. They really may not understand what can be non-kosher about a bagel or some French fries [4.
If the mumar above is not one who violates kashrut what sin can this person habitually violate that we label him a mumar and yet we may eat from his shechita? Rav Anan in the name of Shmuel explains, that we are talking about one who worships idols! But since this idol-worshipper is otherwise 'a nice Jewish boy' who would never knowingly serve somebody non-kosher food and we may eat from the shechita of this idol-worshipper. To say that this is mindboggling does not quite fully capture the radical nature of this view. One might cogently argue that the primary teaching of the Torah is the belief in one G-d. Could it really be that we can eat meat slaughtered by such a person? How many Jews have gone to their deaths with shema yisrael on their lips? While the Gemara will eventually reject this view, declaring the shechita of an idol worshipper treif, what in the world were Rav Anan (and Shmuel) thinking?
Rashi explains that the yetzer hara, the desire in earlier times to worship idols was so strong that many who would never eat non-kosher food succumbed to the temptation of idolatry. Yes, it is possible that one may worship idols and yet keep meticulously kosher allowing Rav Anan argues to eat from their shechita .
We today have trouble relating to such a view. But we live in an era, unlike that of the time period of the Bible, where idolatry offers little attraction. The Talmud (Sanhedrin 64a) claims that the rabbis prayed that G-d rid us of the desire for idolatry, and those prayers were heeded. Rav Anan was apparently reflecting this reality. But as time went on and the desire for idolatry became a distant memory, excuses could no longer be made for idolaters and thus the rabbis declare that “an apostate with regard to idol worship is an apostate with regard to the entire Torah” (Chulin 5a).
 Rabbi Tendler said that undoubtedly the most the most important teshuva Rav Moshe wrote was when as a relatively young posek, Rav Moshe allowed all women survivors of the Holocaust to remarry, despite lack of evidence that their husbands were actually dead. With modern methods of communication—and this is 1947!—if a husband did not contact his wife we could assume it was because he is dead (Even HaEzer 1:43). It is not hard to see how crucially important this teshuva was and how someone of lesser shoulders might not have written such. It is obvious that while this was sadly true in the vast majority of cases, it is also obvious that it was not universally true. In actual fact this teshuva, in addition to not giving the evil ones a posthumous victory, “allowed” the occasional case of unintentional adultery.
 This teshuva, responding to a questioner in Moscow in 1934, reflects the currents of the time where even in the most observant of families most of the children left the world of religion; the list of gedolei Torah whose children were not shomer Shabbat was a long one. Today with so many baalei teshuva we might ask if an observant child can eat at the home of the non observant parent. And yet we must constantly note that despite the successes of the baal teshuva movement the number of those leaving observance far outnumbers those joining.
 Fascinatingly, Rava is of the view that in general the testimony of a mumar lehachis may be accepted (Sanhedrin 27a). Just because one has no respect for the laws of kashrut does not mean that one is not a person of great integrity and meticulously honest. At the same time, Rava agrees that those who do not keep kosher because it’s too expensive cannot have their testimony accepted. If they are willing to ignore their beliefs to save a few dollars, we can have no assurance that their testimony will be free of monetary biases.
 Rav Moshe does not address this issue in his teshuva presumably because having grown up in a kosher home they would know how to prepare a kosher meal if they wanted to. Perhaps one can argue that today with so many foods having actual certification even those totally ignorant of kashrut can prepare a basic kosher meal.