15 Everything that breaketh open the womb of all flesh, which they present to Jehovah, of men or of beasts, shall be thine; nevertheless the firstborn of man shalt thou in any case ransom, and the firstborn of unclean beasts shalt thou ransom. 16 And those that are to be ransomed from a month old shalt thou ransom, according to thy valuation, for the money of five shekels, after the shekel of the sanctuary, which is twenty gerahs. 17 But the firstborn of a cow, or the firstborn of a sheep, or the firstborn of a goat, thou shalt not ransom: they are holy. Thou shalt sprinkle their blood on the altar, and their fat shalt thou burn as an offering by fire for a sweet odour to 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.