The Torah declares of every first-born: "he is Mine." But elsewhere, God commands: "the male shall you sanctify to the Lord your God" (Deuteronomy 15:19). The Mekhilta spots a tension between these two statements and resolves it with a teaching about the nature of divine commandments.
The question is straightforward. If the first-born already belongs to God — "he is Mine," an automatic, inherent sanctity — then why does the Torah also command humans to actively sanctify it? Does our action of dedication make any difference? If we sanctify the animal, it is holy. But if we fail to sanctify it, is it not already holy by divine decree?
The answer: "he is Mine" establishes that the first-born's sanctity exists regardless of human action. The animal is holy whether or not anyone performs a dedication ceremony. Its status is not contingent on us. It belongs to God from the moment it is born.
So what does "the male shall you sanctify" accomplish? It gives the human being an opportunity to earn reward. The act of consciously dedicating the first-born to God — saying the words, performing the ritual, acknowledging God's ownership — does not create the holiness. It creates merit for the person who does it. God does not need our sanctification. We need the act of sanctifying.
The Mekhilta reveals here a remarkable theology of commandments. Many mitzvot (commandments) exist not because God requires the outcome but because humans require the process. The doing is the point. The reward comes from the effort, not from changing a reality that already exists.