Numbers 3:15
Number the children of Levi after the house of their fathers, by their families: every male from a month old and upward shalt thou number them. Number the children of Levi after the house of their fathers, by their families: every male from a month old and upward shalt thou number them.
The verse belongs to the first of the two numberings of Israel that give the book of Numbers its name, the numbering near the beginning of the book as distinct from the numbering near its close, and together with the first four chapters it deals with the numbering of the people, the numbering of the tribe of Levi, and the office of the various families of Levi. It is part of organizing the Levites to care for the tabernacle, the sanctuary of the Lord, by dividing Levi into the families of Gershon, Kohath, and Merari, whose assigned camps were on the west, south, and north sides of the tabernacle. It commands the numbering of the children of Levi after the house of their fathers by their families, counting every male from a month old and upward, older than one month, with definite numbers of men of a certain age given.
The passage shows that the official priests and Levites were distinctly segregated in their relation to the sanctuary and its services, with the two offices in no sense similar or interchangeable though both orders descended from Levi, the labors of Levitical duties falling upon the sons of Moses separate from the priesthood perpetuated through Eleazar and Ithamar after the death of Nadab and Abihu. The chapter is part of the law assigning the Kohathite family of the Levites to carry the ark and sacred vessels when they were moved, to carry them on their shoulders, not to touch them, and to approach them only after the sacred objects had been covered, showing that the ark's transport had to follow God's prescribed order. It shows that the priesthood was assigned by law to the tribe of Levi, under the order of Aaron, for that age, and service in the priest's office by someone from another tribe was forbidden.
The chapter gives the order from the book of Moses for the service of God at Jerusalem, with the priests set in their divisions and the Levites in their courses, and the returned exiles appointed their temple service at the dedication according to this order. The definite numbers given in this numbering are to be referred back to when the numbering at the close of the forty years is reached, to note how the tribes have gained and lost.