And the LORD said unto Moses, Number all the firstborn of the males of the children of Israel from a month old and upward, and take the number of their names.

Jamieson-Fausset-Brown Bible Commentary

40-51. Number all the first-born of the males of the children of Israel, &c.-The principle on which the enumeration of the Levites had been made was now to be applied to the other tribes. The number of their male children, from a month old and upward, was to be reckoned, in order that a comparison might be instituted with that of the Levites, for the formal adoption of the latter as substitutes for the first-born. The Levites, amounting to twenty-two thousand, were given in exchange for an equal number of the first-born from the other tribes, leaving an excess of two hundred seventy-three; and as there were no substitutes for these, they were redeemed at the rate of five shekels for each (Nu 18:15, 16). Every Israelite would naturally wish that his son might be redeemed by a Levite without the payment of this tax, and yet some would have to incur the expense, for there were not Levites enough to make an equal exchange. Jewish writers say the matter was determined by lot, in this manner: Moses put into an urn twenty-two thousand pieces of parchment, on each of which he wrote "a son of Levi," and two hundred seventy-three more, containing the words, "five shekels." These being shaken, he ordered each of the first-born to put in his hand and take out a slip. If it contained the first inscription, the boy was redeemed by a Levite; if the latter, the parent had to pay. The ransom-money, which, reckoning the shekel at half a crown, would amount to 12s. 6d. each, was appropriated to the use of the sanctuary. The excess of the general over the Levitical first-born is so small, that the only way of accounting for it is, by supposing those first-born only were counted as were males remaining in their parents' household, or that those first-born only were numbered which had been born since the departure from Egypt, when God claimed all the first-born as his special property.

Matthew Henry's Concise Commentary

3:40-51 The number of the first-born, and that of the Levites, came near to each other. Known unto God are all his works beforehand; there is an exact proportion between them, and so it will appear, when they are compared together. The small number of first-born, over and above the number of the Levites, were to be redeemed, and the redemption-money given to Aaron. The church is called the church of the first-born, 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 men are the Lord's by creation, and all true christians are his by redemption. Each should know his own post and duty; nor can any service required by such a Master be rightly accounted mean or hard.