15 The first birth of every living thing which is offered to the Lord, of man or beast, is to be yours; but for the first sons of man payment is to be made, and for the first young of unclean beasts. 16 Payment is to be made for these when they are a month old, at the value fixed by you, a price of five shekels by the scale of the holy place, that is, twenty gerahs to the shekel. 17 But no such payment may be made for the first birth of an ox or a sheep or a goat; these are holy: their blood is to be dropped on the altar, and their fat burned for an offering made by fire, a sweet smell to the Lord.
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.