Commentaire sur Le Lévitique 9:7

קרב אל המזבח [AND MOSES SAID TO AARON] GO TO THE ALTAR, for Aaron was diffident and feared to go there. Moses therefore said to him “Wherefore art thou diffident? For this purpose hast thou been selected!” (cf. Sifra, Shemini, Mechilta d'Miluim 2 8).

DRAW NEAR UNTO THE ALTAR. 8. AND AARON DREW NEAR UNTO THE ALTAR AND SLEW THE CALF. It seems to me in accordance with the plain meaning of Scripture that Moses said to Aaron: “Draw near to the north side of the altar and offer there the sin-offering and the burnt-offering, for they are to be slaughtered on the north side of the altar.”²⁵Above, 1:11; 6:18. Moses, however, said it briefly since Aaron already knew [that these offerings were to be slaughtered on the north side of the altar].
Our Rabbis in the Torath Kohanim were, however, prompted by [the use of] these expressions to draw a parable [and to say]:²⁶Torath Kohanim, beginning of Shemini 8. The Rabbis were prompted by the use of the phrase “draw near,” which Ramban explained to mean “draw near to the north side of the altar.” “To what can this be compared? To a human king who married a woman, who in her shyness [did not dare to enter] before him. Her sister then approached her, saying, ‘My sister, why did you enter this matter? Was it not in order that you serve the king? Embolden yourself and go in to serve the king!’ Similarly did Moses say to Aaron: ‘My brother! Why were you chosen to be the High Priest? Was it not so that you minister before G-d? Embolden yourself and come and do your priestly activities.’ Some Rabbis say that Aaron saw the [horned] altar in the form of the bull [which — as stated in Psalms 106: 20 — Israel had worshipped], and he was frightened by it. Then Moses came near and said to him, ‘My brother, Aaron, do not be afraid of that which you fear. Embolden yourself and come near it.’ This is why he said, draw near unto the altar, and [Aaron] drew near unto the altar — with zeal.” The reason for this [apparition which Aaron saw in the altar] was that since Aaron was the holy one of the Eternal,²⁷Psalms 106:16. having no sin on his soul except for the incident of the golden calf, therefore that sin was firmly fixed in his mind, something like that which is said, and my sin is ever before me.²⁸Ibid., 51:5. It thus appeared to him as if the form of the calf was there [in the altar] preventing his [attaining] atonement [through the offerings he was to bring]. That is why Moses said to him, “Embolden yourself so that you should not be of such humble spirit,” for G-d has already accepted his works.²⁹See Ecclesiastes 9:7. Other scholars explain that it was Satan who showed him this apparition, just as they have said there in the Torath Kohanim:³⁰Torath Kohanim, beginning of Shemini 3. “My brother Aaron, although G-d has agreed to grant atonement for your sin, you must nonetheless close the mouth of Satan [through your offerings], lest he accuse you when you enter the Sanctuary etc.” And the expression, and make atonement for thyself, and for the people means as follows: “draw near unto the altar to bring all the offerings, and offer first thy sin-offering, and thy burnt-offering, and make atonement for thyself first with thy offerings, and for the people afterwards, through bringing their offering and atoning for them by means thereof.” Thus Moses taught Aaron that the guiltless should come and effect atonement for those who are guilty.³¹Yoma 43 b.

וכפר בעדך ובעד העם. "and make atonement for yourself and for the people." We learn from this that Aaron's own atonement effected the atonement of the people. This is because Aaron's sin was brought about through the Israelites. Had they not demanded from Aaron that he make a deity for them, none of all this would have happened. As long as Aaron had not obtained atonement for his part in that sin, both the sinner and the one who had been the prime cause of that sin still remained guilty. As soon as Aaron obtained atonement, so did the people. We derive all this from the linkage of the two atonements in this verse i.e. "on your behalf and on behalf of the people."