12 you shall set apart to the Lord all that first opens the womb. All the firstlings of your cattle that are males shall be the Lord's. 13 Every firstling of an ass you shall redeem with a lamb, or if you will not redeem it you shall break its neck. Every first-born of man among your sons you shall redeem. 14 And when in time to come your son asks you, 'What does this mean?' you shall say to him, 'By strength of hand the Lord brought us out of Egypt, from the house of bondage. 15 For when Pharaoh stubbornly refused to let us go, the Lord slew all the first-born in the land of Egypt, both the first-born of man and the first-born of cattle. Therefore I sacrifice to the Lord all the males that first open the womb; but all the first-born of my sons I redeem.'
Matthew Henry's Commentary on Exodus 13:12-15
Commentary on Exodus 13:11-16
(Read Exodus 13:11-16)
The firstlings of beast not used in sacrifice, were to be changed for others so used, or they were to be destroyed. Our souls are forfeited to God's justice, and unless ransomed by the sacrifice of Christ, will certainly perish. These institutions would continually remind them of their duty, to love and serve the Lord. In like manner, baptism and the Lord's supper, if explained and attended to, would remind us, and give us occasion to remind one another of our profession and duty.