What was Hashem's response?
Rashi: Hashem answered that Moshe should descend 1 and warn the people again, due to the principle that one is obligated to warn the people when the prohibition is issued and again when the matter falls due. 2
Rashbam (to 19:22): He now instructed Moshe to descend and to ascend once again together with Aharon; and as for the people, they were forbidden to break their ranks in order to ascend, but not if they only intended to move closer to the mountain.
Da'as Zekenim, Hadar Zekenim and Rosh (all to 19:21): Hashem responded that Moshe and Aharon will ascend more than they; and that consequently, they will not be with them to warn them then, so he should warn them now.
Moshav Zekenim (to 19:23): You and Aharon will ascend, and they will be left like a flock without a shepherd.
Moshe was on such a high level that if Hashem issued a command, it was inconceivable to transgress it, in which case it was not necessary to repeat it. Therefore Hashem said to him "Lech Reid" - Come down from your high level! (See Rashi, Shemos 32:7). Much in the same way as the Gemara in Shabbos (20a) says about the Kohanim 'Kohanim Zerizim Heim.' The people however, are not on that level, and they require a second warning.
If Moshe ascended Har Sinai before the Aseres ha'Dibros were given, and the people did not move from their place, where were Aharon and the 'Kohanim'?
Rashi: The Kohanim accompanied Moshe part of the way and set up their camp, and Aharon accompanied Moshe still further, setting up camp somewhere in between the Kohanim and Moshe. 1
Seforno: Hashem instructed Moshe to descend and to remain with the people while the Aseres ha'Dibros were being said; and it was only after the Aseres ha'Dibros that He ordered Moshe to ascend with Aharon, and the Kohanim and the people to remain in their places. 2
The problem with this is that, a. that the Pasuk implies that Aharon accompanied Moshe to wherever it was that he was going, and b. when Moshe eventually returned holding the Luchos, Aharon was in the camp, together with the people; and it was exclusively Yehoshua, who had accompanied his Rebbe to the outskirts of the camp, who was encamped on his own (Shemos 32:1-6, 32:17-25)? See Oznayim la'Torah on Pasuk 25.
Seforno: See Shemos 24:1 .
QUESTIONS ON RASHI
Rashi writes: "You shall ascend, and Aharon with you, and the Kohanim - ... This means... that Aharon had a boundary for himself; and the Kohanim for themselves... Aharon approached further than the Kohanim." What is Rashi's source for this?
Gur Aryeh #1: Since Moshe would have a partition for himself, 1 due to his greatness, [it follows that] Aharon would have his own place as well.
Gur Aryeh #2: The extra phrase "and Aharon with you" teaches that he would ascend further than the other Kohanim.
Which we learn from the word "Atah."
Rashi writes: "... Whereas the people may not break ranks at all to ascend towards Hashem." Where does the text indicate that the Kohanim would be permitted to ascend further than the people at large?
