1) "MA'ASER" TAKEN FROM THE PRODUCE OF A "CHASHUD"
QUESTION: The Mishnah states that one who is suspected of doing business with fruit of Shevi'is is not suspected of disregarding the laws of separating Ma'aser from his produce. RASHI (DH Eino) explains that one may purchase produce from such a person (in any year other than the Shevi'is year, of course), and it is treated like Demai (produce for which the obligation to separate Ma'aseros is in doubt), and one separates Ma'aser only out of doubt (and thus a number of leniencies apply; see Shabbos 23a). (The CHAZON ISH (Shevi'is 10:7, DH v'Gidro) points out that the words of Rashi here imply that a person who is suspected of not observing the laws of Shemitah is automatically classified as an Am ha'Aretz, and therefore one must always separate Ma'aser from produce purchased from him.)
This implies that if the person is suspected of not separating Ma'aser from his produce, one must separate definite Ma'aser (and not only doubtful Ma'aser) from what is purchased from him.
However, since the person is only suspected of neglecting to separate Ma'aser, why should his produce not be considered Demai, from which one must separate Ma'aser only out of doubt? (TOSFOS REBBI AKIVA on the Mishnah)
ANSWER: Apparently, when a person is suspected of not separating Ma'aseros, the logic that applies to an ordinary Am ha'Aretz is reversed. It is assumed that an ordinary Am ha'Aretz usually separates Ma'aseros; one who purchases his produce from him must separate Ma'aser merely out of doubt. In the case of one who actually is suspected of not separating Ma'aseros, the opposite logic applies: most of the time he does not separate Ma'aser. One who purchases his produce must assume that his produce is definitely Tevel (based on Rov) and must separate Ma'aser with a Berachah from the produce. (M. KORNFELD)
Perhaps this may be understood better based on the words of the BEIS SHAUL (by RAV YOSEF SHAUL NATANSON, the author of SHO'EL U'MESHIV) on the Mishnayos, in his answer to the question of Rebbi Akiva Eiger. He cites TOSFOS (end of 20a) who asks that according to Rebbi Meir, who is always concerned for the minority (Mi'ut), why is the obligation to separate Ma'aser from produce received from an Am ha'Aretz only mid'Rabanan? Even though a majority of Amei ha'Aretz separate Ma'aser, since there is a Mi'ut who do not, Rebbi Meir should rule that the Mi'ut combines with the Chazakah that the produce is still Tevel, and the obligation to separate Ma'aser from it should be mid'Oraisa! Tosfos answers that we cannot rely on the minority of Amei ha'Aretz who do not separate Ma'aser in order to create a situation that makes every Am ha'Aretz into a Rasha who does not separate Ma'aser.
The Beis Shaul explains that the statement of Tosfos applies to an average Am ha'Aretz, for whom we have no particular reason to believe that he does not separate Ma'aser. However, when the Mishnah mentions a "Chashud," it refers to a person for whom we have specific evidence that indicates that he does not separate Ma'aser from his produce. Therefore, according to Rebbi Meir, the obligation to separate Ma'aser from the produce of a "Chashud" is mid'Oraisa, because such a person indeed may be considered a Rasha. Even the Chachamim who disagree with Rebbi Meir agree that mid'Rabanan the Chazakah of Tevel joins with the Mi'ut. Therefore, there is an obligation mid'Rabanan that is Vadai (and not mi'Safek) to separate Ma'aser from the produce of a "Chashud." This obligation is stronger than the Safek d'Rabanan obligation to separate Ma'aser from Demai of an ordinary Am ha'Aretz. (See PNEI YEHOSHUA to Kesuvos 24a, DH Amar Rava, who also distinguishes between a person who is "Chashud" and an ordinary Am ha'Aretz.) (D. BLOOM)
2) A GER WHO REJECTS ONE OF THE MITZVOS
QUESTION: The Gemara states that a Ger who is suspected of consistently transgressing even one Mitzvah is suspected of transgressing the entire Torah. He is given the status of a Yisrael Mumar (an apostate Jew) but not a Nochri (and thus if he is Mekadesh a woman, the Kidushin takes effect).
Why is the Ger treated like a Yisrael Mumar? Perhaps the fact that he is "Chashud" for transgressing the Mitzvos demonstrates that his initial acceptance of the Mitzvos was insincere, and thus he should not be considered a Jew altogether.
ANSWER: The RAMBAM (Hilchos Isurei Bi'ah 13:14-15) writes, "Do not think that Shimshon or Shlomo ha'Melech married Nochri women while they were still Nochrim. Rather, the root of the matter is as follows: The correct procedure is to check a person who is coming to convert to ensure that he is not converting for monetary gain or to gain power or out of fear.... Since Shlomo converted many women and married them, and we know that they were motivated by various material gains and were not converted by a Beis Din, the verse considers them Nochrim and they are prohibited to marry. Furthermore, their later actions proved their initial intentions."
It appears from the words of the Rambam that only when we are certain that a Ger originally converted with sincere intent is he considered a Yisrael Mumar when he rebels. If there is any suspicion that he may have converted for ulterior motives and did not sincerely accept the Mitzvos, then his rebellion demonstrates that he indeed did not accept the Mitzvos when he converted, and retroactively the conversion is invalid.
The DEVAR AVRAHAM (3:28:3) writes that although the Halachah is that "Devarim sheb'Lev Einam Devarim" -- intentions in a person's heart that he has not expressed verbally are not binding, nevertheless when we know that a Nochri converted with ulterior motives his conversion is invalid. Even though the convert verbally declared that he accepts the Mitzvos, since it can be assumed with almost complete certainty ("Umdena d'Muchach") that he did not accept the Mitzvos sincerely, it is considered as though he verbally expressed such a lack of sincere intent.
3) HALACHAH: A CONVERT WHO KNOWS THAT HE WILL TRANSGRESS
OPINIONS: The Gemara makes it clear that in order for a conversion to be valid, the convert must accept all of the Mitzvos, without exception.
What is the status of the conversion in a case in which the prospective convert genuinely and sincerely accepts upon himself to adhere to the Torah and Mitzvos, but in his heart he knows that he will most likely succumb on occasion to his Yetzer ha'Ra? Is his conversion valid?
(a) The ACHIEZER (3:26:4) rules that since such a convert genuinely accepts the Mitzvos upon himself and acknowledges that he is bound to keep them, his conversion is valid even though he knows he will succumb to his Yetzer ha'Ra. However, his conversion is valid only if he has absolute intention to uphold the basic tenets of Torah, but he realizes that occasionally he may fail in some specific regard. If he plans in advance to transgress any basic tenet of Torah, such as Shabbos or Kashrus, on a consistent basis, this is not considered an acceptance of the Mitzvos and the conversion is not valid.
This ruling seems similar to the explanation of the RAMBAN to the verse, "Cursed is he who will not uphold the Torah to perform it" (Devarim 27:26). The RAMBAN explains that this verse obligates a person to acknowledge the truth of the entire Torah. If a person denies the validity of one of the Mitzvos or deems any one of them no longer valid, then "cursed is he." However, if a person sins because of his overpowering lusts but he still accepts the Torah as true, he still is considered one who is blessed because he "upholds the words of this Torah" and the curse in the verse does not apply to him. (Z. Wainstein)
(b) The DEVAR AVRAHAM (3:28) disagrees and maintains that if the convert has in mind to transgress certain Mitzvos because he knows that his Yetzer ha'Ra will overpower him, then his conversion is not valid, since his acceptance of the Mitzvos is lacking.