Verse 17. - But the firstling of a cow, etc. Only those things which were not available for sacrifice could be redeemed; the rest must be offered to him that claimed them. The first-born of men belonged partially to both classes: on the one hand, they could not be sacrificed, and therefore were redeemed with money; on the other hand, they could be dedicated (being clean), and therefore had been exchanged for the Levites.

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.But the firstling of a cow, or the firstling of a sheep, or the firstling of a goat, thou shalt not redeem,.... That is, shalt not take the redemption price for them, but the creatures themselves; which they, the priests, were to take for their own use:

they are holy; separated to the Lord, and therefore not to be used in common by men, but were sacred to the Lord, or became the property of his priests; see Deuteronomy 15:19; how they were to be made use of follows:

thou shalt sprinkle their blood upon the altar; for they were to be sacrificed, and their blood used as in other sacrifices:

and shalt burn their fat for an offering made by fire; for a sweet savour unto the Lord; as the fat of other sacrifices was, and particularly of the peace offerings, which were wholly eaten otherwise, Leviticus 7:31.