Jamieson, Fausset and Brown on Leviticus 8:3-5

3-5. gather thou all the congregation together, &c.--It was manifestly expedient for the Israelitish people to be satisfied that Aaron's appointment to the high dignity of the priesthood was not a personal intrusion, nor a family arrangement between him and Moses; and nothing, therefore, could be a more prudent or necessary measure, for impressing a profound conviction of the divine origin and authority of the priestly institution, than to summon a general assembly of the people, and in their presence perform the solemn ceremonies of inauguration, which had been prescribed by divine authority.