Matthew Henry Commentary Numbers 3:40-51

"And Jehovah said unto Moses, Number all the first-born males of the children of Israel from a month old and upward, and take the number of their names. And thou shalt take the Levites for me (I am Jehovah) instead of all the first-born among the children of Israel; and the cattle of the Levites instead of all the firstlings among the cattle of the children of Israel: and Moses numbered, as Jehovah commanded him, all the first-born among the children of Israel. And all the first-born males according to the number of names, from a month old and upward, of those that were numbered of them, were twenty and two thousand two hundred and threescore and thirteen. And Jehovah spake unto Moses, saying, Take the Levites instead of all the first-born among the children of Israel, and the cattle of the Levites instead of their cattle; and the Levites shall be mine: I am Jehovah. And for the redemption of the two hundred and threescore and thirteen of the first-born of the children of Israel, that are over and above [the number of] the Levites, thou shalt take five shekels apiece by the poll; after the shekel of the sanctuary shalt thou take them (the shekel is twenty gerahs): and thou shalt give the money, wherewith the odd number of them is redeemed, unto Aaron and to his sons. And Moses took the redemption-money from them that were over and above them that were redeemed by the Levites; from the first-born of the children of Israel took he the money, a thousand three hundred and threescore and five [shekels], after the shekel of the sanctuary: and Moses gave the redemption-money unto Aaron and to his sons, according to the word of Jehovah, as Jehovah commanded Moses." — Numbers 3:40-51 (ASV)

The number of the firstborn, and that of the Levites, were close to each other. Known to God are all His works beforehand; there is an exact proportion between them, and so it will appear when they are compared. The small number of firstborn, exceeding the number of the Levites, were to be redeemed, and the redemption money was to be given to Aaron.

The church is called the church of the firstborn, which is redeemed, not as they were, with silver and gold; but, being devoted by sin to the justice of God, is ransomed with the precious blood of the Son of God. All people are the Lord's by creation, and all true Christians are His by redemption. Each should know their own post and duty; nor can any service required by such a Master be rightly considered insignificant or difficult.