12 you must present all firstborn sons and firstborn male animals to the Lord, for they belong to him. 13 A firstborn donkey may be bought back from the Lord by presenting a lamb or young goat in its place. But if you do not buy it back, you must break its neck. However, you must buy back every firstborn son. 14 "And in the future, your children will ask you, 'What does all this mean?' Then you will tell them, 'With the power of his mighty hand, the Lord brought us out of Egypt, the place of our slavery. 15 Pharaoh stubbornly refused to let us go, so the Lord killed all the firstborn males throughout the land of Egypt, both people and animals. That is why I now sacrifice all the firstborn males to the Lord -except that the firstborn sons are always bought back.'
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.