Shalt thou redeem - for the money of five shekels - Redemption of the first-born is one of the rites which is still practiced among the Jews. According to Leo of Modena, it is performed in the following manner: -

When the child is thirty days old, the father sends for one of the descendants of Aaron: several persons being assembled on the occasion, the father brings a cup containing several pieces of gold and silver coin. The priest then takes the child into his arms, and addressing himself to the mother, says: Is this thy son?

Mother. Yes.

Priest. Hast thou never had another child, male or female, a miscarriage or untimely birth?

Mother. No.

Priest. This being the case, this child, as first-born, belongs to me.

Then, turning to the father, he says: If it be thy desire to have this child, thou must redeem it.

Father. I present thee with this gold and silver for this purpose.

Priest. Thou dost wish, therefore, to redeem the child?

Father. I do wish so to do.

The priest then, turning himself to the assembly, says: Very well; this child, as first-born, is mine, as it is written in Bemidbar, (Numbers 18:16), Thou shalt redeem the first-born of a month old for five shekels, but I shall content myself with this in exchange. He then takes two gold crowns, or thereabouts, and returns the child to his parents.

And those that are to be redeemed,.... Which is to be understood not of unclean beasts last mentioned, for they were redeemed with lambs and not with money; and after they were eight days and not a month old, as follows, Exodus 13:13; but of the firstborn of men:

from a month old shalt thou redeem, according to thine estimation: that is, take the redemption money next mentioned; and though both time and price are fixed, yet said to be according to the estimation of the priest; see Leviticus 27:5,

for the money of five shekels; which was about eleven or twelve shillings of our money, and is the price given for the redemption of the firstborn, when the Levites were taken in their stead, Numbers 3:47,

after the shekel of the sanctuary, which is twenty gerahs; see Gill on Numbers 3:47; of the manner of redeeming the firstborn; see Gill on Exodus 13:15.

And those that are to be redeemed from a month old shalt thou redeem, according to thine estimation, for the money of five shekels, after the shekel of the sanctuary, which is twenty gerahs.

18:16 Those that are to be redeemed - Namely, of men only, not of unclean beasts, as is manifest from the time and price of redemption here mentioned, both which agree to men; the time, Num 18:16, the price, Num 3:46,47, but neither agree to unclean beasts, which were to be redeemed with a sheep, Exo 13:13, and that after it was eight days old.

18:8-19 All believers are spiritual priests, and God has promised to take care of them. Godliness has the promise of the life that now is. And from the provision here made for the priests, the apostle shows that it is the duty of christian churches to maintain their ministers. Scandalous maintenance makes scandalous ministers. The priests were to be wholly devoted to their ministry, not diverted from it, or disturbed in it, by worldly care or business. Also, that they might be examples of living by faith, not only in God's providence, but in his ordinances. The best should be offered for the first-fruits unto the Lord. Those who think to save, by putting God off with the refuse, deceive themselves, for God is not mocked.