A more literal translation would state that God had made
the firstborn holy,
or that he had sanctified
them. This stands in direct contrast to the firstborn of Egypt who were put to death on account of their wickedness. The lives of the firstborn in Israel were spared—not on account of their own intrinsic righteousness, but on account of the blood of the paschal lamb (Exodus 12:1–51). As such, God sanctified them, consecrated them, set them apart for himself. He did so on his terms, for his glory alone.
13 Car tout premier-né m'appartient; le jour où j'ai frappé tous les premiers-nés dans le pays d'Egypte, je me suis consacré tous les premiers-nés en Israël, tant des hommes que des animaux: ils m'appartiendront. Je suis l'Eternel.