12 you shall set apart to the Lord all that first opens the womb. All the firstborn of your animals that are males shall be the Lord's. 13 Every firstborn of a donkey you shall redeem with a lamb, or if you will not redeem it you shall break its neck. Every firstborn 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 a strong hand the Lord brought us out of Egypt, from the house of slavery. 15 For when Pharaoh stubbornly refused to let us go, the Lord killed all the firstborn in the land of Egypt, both the firstborn of man and the firstborn of animals. Therefore I sacrifice to the Lord all the males that first open the womb, but all the firstborn 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.