The Gemara in Sanhedrin 58b says:
עובד כוכבים ששבת חייב מיתה שנא’ ויום ולילה לא ישבותו
A non-Jew that keeps Shabbat is liable to the death penalty as it is written “day and night he shall not rest”.
This seems quite radical and I think its good to remind oneself that when saying death penalty its likely to be an exaggerated form of speech, its not like they had the power to carry out such sentences.
Nonetheless it remains surprising, what is so bad about a non-Jew keeping Shabbat?
A Midrash has this Gemara in a slightly different fashion:
עובד כוכבים ששמר את השבת עד שלא קבל עליו את המילה חייב מיתה
A non-Jew that keeps the Shabbat is liable to the death penalty until the point that he accepts circumcision.
The addition is quite significant though, does this mean they should be converted first? The continuation of the Midrash however implies that this is not referring to conversion because it asks: “Why? They arent commanded on it?”
Rabbi Yechiel Yaakov Weinberg read a very interesting explanation of this midrash which he relates in his piece titled: “Whether its permissable to give a lecture on the judgements (laws) of Israel in front of a non-Jewish audience”.
He says:
ראיתי לפני שלשים שנה ויותר בספר אחד שכתב בשם חכם אחד כי המאמרים בסנהדרין על עכו”ם ששבת ועל העוסק בתורה נאמרו נגד כת הנוצרים הראשונים שפרקו מעליהם ברית מילה אבל קיימו את השבת וגם עסקו בתורה והיו מזדווגן לבנ”י בשבת ומועד ושמעו כל דבריהם ואח”כ הלכו והלשינו עליהם בפני הרשות
More than 30 years ago I saw in a book written by a certain Rabbi that what they say in Sanhedrin on a non-Jew that keeps Shabbat and engages in Torah (that he is liable to the death penalty) was said regarding a sect of early Christians who cast off from themselves [the commandment] of circumcision, but fulfilled the Shabbat and also engaged in Torah, and they joined together with the Jews on Shabbat and holidays and they heard all of their words and afterwards went and tattled/informed on them with the government.
So according to this author the Gemara was reacting towards a particular series of events that took place in synagogue(s). This group of early Christians that were to some extent observing Torah were joining them – perhaps every once in a while or continually – on Shabbat and some day told the government about what the Jews were discussing, presumably phrased in ways that the government would be less pleased with.
Thats why the Midrash adds “until the point that he accepts circumcision” because that ensures the Gemara’s statement is restricted to that sect of early Christians and not to all non-Jews that desire to keep Shabbat. The obstacle is however quite significant, which ensures at the same time that it is accessible only to those that are very dedicated or those that are lucky to be circumcized by their parents already (though Im not sure this was common in the time of the Gemara like it is today).
I seem to recall this also the other way around, where the Jews were at a certain point in time distancing themselves from the early Christians before the government which was not appreciated by the early Christians. Perhaps someone recalls the event Im referring to. I think this is a rather interesting example also of the relatively late parting of the ways between Christianity and Judaism. Im not sure whether it can be imagined that (dedicated) Jews were attending the meetings of early Christians (in whatever fashion these took place) but the other way around seems certainly to have been the case even if it ended in this particular instance in a negative way.