15 Everything that openeth the womb, of all flesh which they offer unto Jehovah, both of man and beast shall be thine: nevertheless the first-born of man shalt thou surely redeem, and the firstling of unclean beasts shalt thou redeem. 16 And those that are to be redeemed of them from a month old shalt thou redeem, according to thine estimation, for the money of five shekels, after the shekel of the sanctuary (the same is twenty gerahs). 17 But the firstling of a cow, or the firstling of a sheep, or the firstling of a goat, thou shalt not redeem; they are holy: thou shalt sprinkle their blood upon the altar, and shalt burn their fat for an offering made by fire, for a sweet savor unto Jehovah.
Matthew Henry's Commentary on Numbers 18:15-17
Commentary on Numbers 18:8-19
(Read Numbers 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.