Exodus 19:22

Sanctified, in an extraordinary manner, above the rest. These priests, according to St. Augustine, are the children of Aaron, and the whole race of Levi, who would shortly be selected by God. But others think, they are those who, by the law of nature, were accustomed to officiate. Or, as God had declared that they were all a priestly kingdom, some of the most comely and irreproachable youths of each family, had been chosen to present victims, when the covenant with God was to be ratified, chap. xxiv. 15. (Calmet)