Moses at the census did not take into consideration the tribe of Levi, because God had not commanded him to select a prince for this tribe as for all others, hence he drew the conclusion that they were not to be counted. Naturally he was not sure of his decision in this matter, and wavered whether or not to include the Levites in the number, when God said to him: "Do not muster the tribe of Levi, nor number them among the children of Israel." At these words Moses was frightened, for he feared that his tribe was considered unworthy of being counted with the rest, and was therefore excluded by God. But God quieted him, saying: "Do not number the Levites among the children of Israel, number them separately." There was several reasons for numbering the Levites separately. God foresaw that, owing to the sin of the spies who were sent to search the land, all men who were able to go to war would perish in the wilderness, "all that were numbered of them, according to their whole number, from twenty years old and upward." Now had the Levites been included in the sum total of Israel, the Angel of Death would have held sway over them also, wherefore God excluded them from the census of all the tribes, that they might in the future be exempt from the punishment visited upon the others, and might enter the promised land. The Levites were, furthermore, the body-guard of God, to whose care the sanctuary was entrusted - another reason for counting them separately. God in this instance conducted Himself like the king who ordered one of his officers to number his legions, but added: "Number all the legions excepting only the legion that is about me." 
The extent of God's love for Levi is evident through the command given to Moses, to number in the tribe of Levi "all males from a month old and upward," whereas in the other tribes none were numbered save men able to go to war, from twenty years and upward. Upon other occasions God had even the embryos among the Levites numbered. This occurred upon Jacob's entrance into Egypt, when the number seventy for his family was attained only by including Jochebed who was still in the womb; and similarly at a future time upon the return of the exiles from Babylon. For at that time only twenty-three of the priestly sections returned, hence to complete their number they had to include Bigvai, who belonged to the missing section, even though he was still in the womb. 
When Moses was ordered to number among the Levites all children from a month old and upward, he said to God: "Thou biddest me count them from a month old and upward. Shall I now wander about their courts and houses and count each child, seeing that Thou givest me such a command?" But God replied: "Do thou what thou canst do, and I will do what I can do." It now came to pass that whenever Moses betook himself to a Levite tent he found the Shekinah awaiting him, tell him exactly the number of children without his having to count them. 
In the choice of this tribe God showed His preference for the seventh, for Levi was the seventh pious man, starting from Adam, to wit: Adam, Noah, Enoch, Abraham, Isaac, Jacob, and Levi. As in this instance, so in many others did God indicate His love for the seventh. He sits enthroned in the seventh heaven; of the seven worlds the seventh alone is inhabited by human beings; of the early generations the seventh was the most excellent, for it produces Enoch. Moses, seventh among the Patriarch, was judged worthy of receiving the Torah. David, seventh son of Jesse, was chosen as king. In periods of time, also, the seventh was the favorite. The seventh day is the Sabbath; the seventh month, Tishri, is the month of the holy days; the seventh year is the Sabbatical year of rest, and every seventh Sabbatical year of rest is the year of jubilee. 
Another reason for numbering even the youngest boys among the Levites was that the tribe of Levi as a whole had the responsibility of atoning for the sin of the first-born among the children of Israel. For it was these who until the time of the worship of the Golden Calf performed the services of the priesthood, and their privilege was taken from them owing to this, their sin. This prerogative was then conferred upon the tribe of Levi, who, moreover, dedicating themselves, man for man, to the service of the Lord, served as an atonement for the first-born of Israel, that they might not be destroyed as they deserved. 
The exchange of Levites in place of the first-born did, however, present a difficulty. For God had communicated the number of Levites to Moses in the following way: "Their number amounts to as many as the number of My legion." For, when God came down upon Sinai, twenty-two thousand angels surrounded Him, and just as many men did the Levites number. Outside of these there were three hundred first-born among the Levites that could not well be offered in exchange for the first-born among the other tribes, because their standing was the same as theirs. As the number of first-born among the other tribes exceeded the number of Levites by two hundred seventy-three, this surplus remained without actual atonement. Hence God ordered Moses to take from them five shekels apiece by the poll as redemption money, and give it to the priests. The sum was fixed upon by God, who said: "Ye sold the first-born of Rachel for five shekels, and for this reason shall ye give as redemption money for every first-born among ye five shekels."
To avoid quarrels among the first-born, as otherwise each one would try to lay the payment of redemption money upon his neighbor, Moses wrote upon twenty-two thousand slips of paper the word "Levi," and upon two hundred seventy-three the words "five shekels," all of which were then thrown into an urn and mixed. Then every first-born had to draw one of the slips. If he drew a slip with "Levi" he was not obliged to remit any payment, but if he drew "five shekels," he had to pay that sum to the priests.