12 you shall set apart to the Lord all that first opens the womb. All the firstborn of your livestock that are males shall be the Lord's. 13 But every firstborn donkey you shall redeem with a sheep; if you do not redeem it, you must break its neck. Every firstborn male among your children you shall redeem. 14 When in the future your child asks you, "What does this mean?' you shall answer, "By strength of hand the Lord brought us out of Egypt, from the house of slavery. 15 When Pharaoh stubbornly refused to let us go, the Lord killed all the firstborn in the land of Egypt, from human firstborn to the firstborn of animals. Therefore I sacrifice to the Lord every male that first opens the womb, but every 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.