"Those who profane it shall be put to death" — the Torah states the punishment for violating the Sabbath. But the Mekhilta asks: from this verse, we know only the punishment. Where is the prior warning — the exhortation that must precede any punishment in Jewish law?
The principle is fundamental: no one can be punished for violating a commandment unless they were first warned. The Torah must provide both the prohibition (do not do X) and the penalty (if you do X, the consequence is Y). Without a clear prohibition, the punishment cannot be imposed.
The warning against Sabbath labor comes from (Exodus 20:10): "And the seventh day is Sabbath to the Lord your God. You shall not perform any labor." This verse provides the exhortation — the explicit "do not." The death penalty verse provides the consequence.
Together, the two verses form a complete legal framework: warning and punishment, prohibition and penalty. Neither can function without the other. The Mekhilta insists that even the most severe punishment in the Torah — death for Sabbath violation — requires a clearly stated prior prohibition. God does not punish by surprise. He warns first, then holds accountable. The pairing of exhortation and penalty is not optional — it is constitutionally required for every enforceable commandment in the Torah.