Numbers 8:19

And I have given the Levites as a gift to Aaron and to his sons from among the children of Israel, to do the service of the children of Israel in the tabernacle of the congregation, and to make an atonement for the children of Israel: that there be no plague among the children of Israel, when the children of Israel come nigh unto the sanctuary.

To make an atonement ... - to aid the priests in that expiatory work; or, as the words may be rendered, 'to make redemption for,' the Levites being exchanged or substituted for the firstborn, for this important end, that there might be a sanctified body of men appointed to guard the sanctuary, and not allow the people to approach or presumptuously meddle with holy things, which would expose them to the angry judgments of heaven.