The Rambam says in the Yad HaChazakah (הלכות איסורי ביאה פרק יג);
(הלכה א') בשלושה דברים נכנסו ישראל לברית: במילה וטבילה וקרבן.
(הלכה ב') מילה היתה במצרים, כשנאמר: "כל ערל לא יאכל בו" (שמות יב מח). מל אותם משה רבנו, שכולם ביטלו ברית מילה במצרים חוץ משבט לוי. ועל זה נאמר: "ובריתך ינצרו" (דברים לג ט).
“The benei yisrael entered into a covenant with Hashem through three things; Bris milah, tevilah and bringing a korban.
The bris milah was in Mitzraim. Because the halacha is that an arel cannot eat from the Korban Pesach, Moshe commanded the benei yisrael to perform bris milah, because all of them stopped doing bris milah in Mitzraim besides the tribe of Levi.”
The Ramban in Yevamos (46a) asks;
אם כן שבט לוי איך נכנסו תחת כנפי השכינה ? אלא הטיפו ממנו דם-ברית. ולי נראה דמדין מילה אינם חייבים להטיף - דהא מלו, ולא דמי לערבי מהול וגבעוני מהול, כיוון דלא מפקדי כמאן דלא מלו דמי
“If the tribe of Levi did bris milah before Moshe commanded them, how did they enter into the covenant with Hashem?
You must say that they did hataphas dam bris.
However this would not have counted towards them entering into a bris with Hashem because they already had bris milah and doing a further hataphas dam bris would not have added anything.
This is not similar to a non-Jew who already has a bris who performs hataphas dam bris if they become a ger, because there seeing as they were not previously commanded to do bris milah it is as if they have not had milah at all so the hataphas dam bris that they do is a new entry into a covenant with Hashem.
However in the case of the Leviim in Mitzraim, seeing as the bris that they already had was a bris that was performed because of a mitzvah, doing hataphas dam bris would not have added anything and could not have been a sign of them entering into a new covenant with Hashem.”
· If so, according to the Rambam, how did the Leviim enter into a bris with Hashem at the time of Yetzias Mitzraim?
The gemara says in Shabbos (87b);
…דכתיב (דברים ה, יא) כאשר צוך ה' אלהיך ואמר רב יהודה אמר רב כאשר צוך במרה
“The passuk says שָׁמוֹר אֶת יוֹם הַשַּׁבָּת לְקַדְּשׁוֹ כַּאֲשֶׁר צִוְּךָ ה אֱלֹקֶיךָ – ‘Guard the day of the Shabbos to sanctify it was Hashem your G-d commanded you.’ Rav Yehudah said in the name of Rav, ‘This means as Hashem commanded you in Marah.’”
The gemara says in Shabbos (118b);
אמר רב יהודה אמר רב אלמלי שמרו ישראל שבת ראשונה לא שלטה בהן אומה ולשון שנאמר (שמות טז, כז) ויהי ביום השביעי יצאו מן העם ללקוט וכתיב בתריה ויבא עמלק
“Rav Yehudah said in the name of Rav – had the benei yisrael only kept the first Shabbos then no nation or language would ever have been able to rule over them, as the passuk says, ‘And it was on the seventh day that some of the people went out to gather man,’, and it says after this, ‘And Amalek came…’”
· If the benei yisrael were already commanded to keep Shabbos in Marah then the Shabbos when the people went out to gather man was not the first Shabbos. Why then does the gemara (118b) say that the benei yisrael did not keep the first Shabbos?
The Maharal (Shemos 15:25) answers as follows;
1) The Rambam says that the reason that we keep bris milah and gid hanashe is not because these mitzvos were given to Avraham and to Yaakov. Rather we keep them because they were re-commanded to Moshe at Har Sinai.
We see from this that there are two types of mitzvah, there are mitzvos that are given through a Navi, such as the sheva mitzvos benei noach, and there are mitzvos that are part of the Torah. Even if the mitzvah is ultimately included as part of the Torah and becomes a mitzvas ha’Torah, it may still have been a mitzvas navi from the time that it was originally commanded until matan torah.
2) Even if a mitzvah is part of the Torah, it is only Torah sheh’biksav if the details of the mitzvah are explicit in the Torah. For example, the passuk says in Devarim (12:21);
וְזָבַחְתָּ מִבְּקָרְךָ וּמִצֹּאנְךָ אֲשֶׁר נָתַן ה לְךָ כַּאֲשֶׁר צִוִּיתִךָ
“And you should slaughter from your cattle and your flocks that Hashem has given to you as I have commanded you.”
We learn from this passuk that there are Hilchos of shechitah that were given to Moshe. However even although there is a reference to Hilchos shechitah in the passuk, they still remain torah shebaal peh because the Torah did not specify the details of any of these halachos.
The only reason that we know that the benei yisrael were commanded concerning Shabbos in Marah is because the passuk says in Devarim - שָׁמוֹר אֶת יוֹם הַשַּׁבָּת לְקַדְּשׁוֹ כַּאֲשֶׁר צִוְּךָ ה אֱלֹקֶיךָ.
The Maharal explains that this ‘kaasher tzivechah’ is not better than the reference of ‘ka’asher tzivischah’ concerning Hilchos shechitah. Just as Hilchos shechitah are not torah shebikesav, so too, from this reference to Shabbos, the mitzvah to keep Shabbos given in Marah could only be at best Torah shebaal peh but not Torah shebiksav.
3) Therefore, had the mitzvah to keep Shabbos that was given in Marah been part of the Torah (a mitzvah of the Torah and not just a mitzvah given by a navi), then it would turn out that the torah shebaal peh of Shabbos came before the torah shebiksav of Shabbos (Shabbos is only referenced explicitly later in the parshah of man.)
However, torah shebaal peh is an explanation of torah shebiksav and not the other way round. Therefore it is impossible that the mitzvah to keep Shabbos given in Marah was a mitzvas hatorah and it must only have been given to the benei yisrael as a mitzvas navi (like gid hanashe when it was commanded to Yaakov).
4) When the gemara in Shabbos (118b) says that the first Shabbos that the benei yisrael had was the Shabbos when they were given the man, it means that this was the first Torah commanded Shabbos that they had. The previous command that they had been given to keep Shabbos in Marah was not part of the giving of the Torah but was rather a mitzvas navi that was given to them in order to make them used to keeping Shabbos.
Rb Yitzchak Hutner z”l (Pesach 36) explains that you see from this Maharal that if a mitzvah was commanded before Har Sinai, then as long as it was written in the Torah it was a mitzvos ha’Torah even before matan torah at Har Sinai.
Therefore you can say that according to the Rambam the Leviim were able to do hataphas dam bris in order to be nichnas tachas kanphei ha’shechinah because the mitzvah that Moshe gave to the benei yisrael in Mitzraim to do milah was a Torah mitzvah that superseded the mitzvas navi to do milah that was given to Avraham.
Therefore with respect to the mitzva of Moshe, the Leviim did not have the status of having milah and were able to do hataphas dam bris.