NOTES for Exo 13:1-22
If we follow the logic of the text given in the Book of Exodus and placed by its author in Moses' mouth, the custom of dedicating everything firstborn to God and redeeming the firstborn child goes back to the celebration of Pesach that took place on the eve of the Exodus. It is hard to say how ancient the custom itself is. Moses may not have been establishing it, but only reminding the people of something once known and now, in Egypt, thoroughly forgotten.
In any case, its connection with the understanding of the feast reflected in the Book of Exodus is obvious. Pesach itself is understood here as a feast of life, or more precisely, as a feast of the triumph of life over death. God's life, because it is God who frees His people from the sword of the angel of death who struck the houses of the Egyptians. Life belongs to God, and to Him alone: such is the meaning of the custom established or restored by Moses.
If so, then it is clear: the first fruits, the first offspring of the livestock, everything first, belongs to God. This is not only so that, as people commonly think, He would then sanctify what follows: the fruits that come after the firstfruits, or the livestock born after the first offspring. Life simply has to return to its first source, to the One who gave it. The only question is what to do here about the human being. With plants and animals it is clear: they were created by God, as the Bible says, "according to their kind," according to the divine design proper to them. One prototype for each genus, species, and subspecies, and the life of an individual creature is not so important.
On the other hand, the whole offspring can then be sanctified at once by sanctifying one sacrificial animal, because animals of one genus, species, and subspecies, especially individuals from one herd, share one life among them. But it is not so with human beings: here each person is unique. People, even those belonging to one clan or tribe, cannot be sanctified like animals. Besides, human beings do not share one life among all; each has his own. What then should be done? The answer is simple: a human being is sanctified in a human way, that is, consciously, as a human being is supposed to be sanctified through sacrifice, by means of a sacrificial meal. But an infant is sanctified by his parents. They are the ones who let God into their life and into the life of their child.
Animals of one species have one soul for all: one is offered in sacrifice, and all are sanctified. Human souls are distinct, but in spouses their souls are joined into one, and the soul of the child enters the circle of their communion. When the parents let God into their shared life, He sanctifies not only their life but also the life of their firstborn, within the single space of relationships that binds the child to the parents and the parents to each other. This is how a human being is sanctified: by letting God into his life.