15 Every firstborn that is offered to God, whether animal or person, is yours. Except you don't get the firstborn itself, but its redemption price; firstborn humans and ritually clean animals are bought back and you get the redemption price. 16 When the firstborn is a month old it must be redeemed at the redemption price of five shekels of silver, using the standard of the Sanctuary shekel, which weighs twenty gerahs. 17 "On the other hand, you don't redeem a firstborn ox, sheep, or goat - they are holy. Instead splash their blood on the Altar and burn their fat as a Fire-Gift, a pleasing fragrance to God.
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.