Sanhedrin 58b: A non-Jew who keeps Shabbos is liable to the death penalty, as the Torah says, “Day and night they shall not rest” (Bereishis 8:22). Ravina said: Even on a Monday.
סנהדרין נח ע”ב אמר ריש לקיש: נכרי ששבת ־ חייב מיתה, שנאמר (בראשית ח׳) ויום ולילה לא ישבתו. ואמר מר: אזהרה שלהן זו היא מיתתן. אמר רבינא: אפילו שני בשבת.
Rabbi Yehosef Rottenberg, rav of Kasan, discussed the problem of how the Avos kept Shabbos, according to those quoted in the Parshas Derachim who hold that the Avos had the status of Jews only as a chumra, not where it conflicted with their obligations as non-Jews.
He proposed that they would drag objects on the ground in such a way that they may or may not make a furrow in the earth. The halacha is like Rabbi Shimon who permits this (Shabbos 29a), since the person does not mean to make the furrow; the furrow is an unintended side effect of his act. The Torah says מלאכת מחשבת which means that an act of work has to be intentional to be considered violation of Shabbos. Thus, as Jews they were not violating Shabbos. However, since in the end the furrow was made, as non-Jews they were not considered to be keeping Shabbos. The rule of מלאכת מחשבת does not apply to non-Jews, since it is learnt from the Mishkan.
On a humorous note, Rabbi Rottenberg reads this into the posuk וישב יצחק בגרר, and Yitzchok rested in Gerar (Bereishis 26:6). He rested – that is, kept Shabbos – by being gorer (dragging) objects on the ground.
Source: Idis Sheb’idis, Toldos
[Some problems with this Dvar Torah:
- Why shouldn’t מלאכת מחשבת apply to non-Jews, just because it is learnt from the Mishkan? Aren’t all the 39 Melachos learnt from the Mishkan?
- If Yitzchok Avinu dragged the bench once and failed to produce a furrow in the ground, he would have had to drag it again, to make sure to violate Shabbos as a non-Jew. If so, he was really intending to produce the furrow, and the heter of “an unintentional act” would not apply.
Now, one might say that this is only a cute “vertel” and there is no point in asking analytical questions. However, it is in fact very relevant to people who have a doubt about whether they are Jewish. The Gemara speaks about such a case in Kesubos 15b: a baby found abandoned in a city where half the residents are Jewish and half are non-Jewish. In our own time, we have the descendants of Marranos and the Ethiopian Jews who are not able to prove their Jewish status. How should such people deal with keeping Shabbos?
Many solutions have been written, but the simplest approach is that of the Binyan Tzion 126: that the rule that a non-Jew may not keep Shabbos has nothing to do with the 39 Melachos. It simply means that he may not rest. For example, carrying heavy furniture around the house is hard work, but it doesn’t violate any of the 39 Melachos. If a non-Jew carries furniture, he is not resting and is therefore doing nothing wrong. This is implied by Rashi who says נכרי ששבת – ממלאכתו – if a non-Jew rests “from his work” – not from the 39 Melachos, but from his own job. Thus the Avos, and someone unsure whether he is Jewish, could simply do some job on Shabbos (e.g. being a waiter at a kiddush, or a household chore) that does not involve any violation of the 39 Melachos.
Alternatively, R’ Moshe Sternbuch (v. 2, 513) writes that a non-Jew studying in preparation for geirus is allowed to keep Shabbos fully, because the prohibition is only on someone who keeps Shabbos as a non-Jew, thinking that he can make up his own religion and receive reward for it. This is based on the Rambam (Melachim 10:9): “The general rule is that we do not permit him to make up laws or do mitzvos for themselves, on their own. Rather, he should either convert to Judaism and accept all the mitzvos, or stay as he is, neither adding nor subtracting.” Here, the prospective convert is keeping Shabbos for the purpose of practice only, so it is okay. The same logic would apply to a person unsure if he is Jewish: he is keeping Shabbos on the possibility that he is Jewish, not under an illusion that he will be rewarded for it if he is not.]