Another explanation of "This is what you shall do to them": It is written, 'For ever, O Lord, Your word stands fast in heaven" (Ps. 119:89). Does then the word of God stand fast only in heaven, but not on earth? Rabbi Hezekiah ben Hiyya said: This is because God made a promise in heaven, which was fulfilled on earth for Abraham after 210 years. How so? When the Holy One, Blessed be He, said to Abraham: Go, get out of your country . . . and I will make you into a great nation (Gen. 12:1), the latter replied: 'Lord of the Universe! What benefit do I derive from all these blessings since I am about to depart from this world childless?' God said to him, "Are you sure that you will no longer give birth to a child?" Abraham replied, "Lord of the Universe! My horoscope tells me that I will be childless.' "So you are afraid of your horoscope?" God retorted. "As you live, it will be as impossible to number your offspring as it is to number the stars of heaven." Rabbi Judah ben Rabbi Simon said in the name of Rabbi Hanin: It was then that God raised Abraham above the vault of the heavens and said to him, "Look now toward heaven, and count the stars, if you can, and God said to him: so shall your seed be (Gen. 15:5), that is: Just as you see all these stars and can not count them, so numerous will your children be, for none will be able to count them . . . This also you now find in the case of Aaron: God made a promise to Moses, saying, "And bring near Aaron your brother . . . that they may minister unto Me in the priests' office" (Exod. 28:1), an assurance which God kept when God said, "And this is what you shall do to them in consecrating them to serve Me as priests."
From Aaron back to Abraham. What happens to all of the promises God makes? How do we trust God's word at a point in the narrative when we're wandering in the dessert, waiting at the foot of the mountain for Moses to relay God's word and reveal our future? When we're wandering through our own adolescent and mid- and late-life crises, waiting for some revelation of our own? As the narrative flow of the Humash pauses to make room for a description of the priestly rites, the midrash takes us back to the powerful stories of Genesis to raise the question made possible by the phrasing of Exodus 29:1: "This is what you shall do to them."
The comfort given is to bring us back into the past, into the story we know and hold dear of Abraham and Sarah's own childlessness coming to an end. The moral is that answers will be personally revealed, as happened with Abraham. Rabbi Neil Gillman, Aaron Rabinowitz and Simon H. Rifkind Professor Emeritus of Jewish Philosophy at The Jewish Theological Seminary, writes in Sacred Fragments about people's need to experience personally revelatory moments as "proof" of revelation; that seems to be the point of the story about God lifting Abraham up to the heavens to see the stars. Abraham saw it with his own eyes and was comforted, and turned back to faith in God instead of faith in his horoscope. So too we, a nation of priests, are to glean comfort in our wanderings—our wanderings through the priestly parashiyot and our wanderings through life—that our own moments of insight and revelation will sustain us, and that we can draw from the experiences of our ancestors to steel us for our journeys ahead.