Sforno on Leviticus 20:27/ספורנו על ויקרא כ׳:כ״ז

1ואיש או אשה כי יהיה בהם אוב או ידעוני, seeing that the thrust of all the preceding legislation had been to sanctify the Jewish people through their observance of these laws, anyone deviating from these commandments, turning instead to these oracles, is doing precisely the opposite of what the Torah wanted him to do. No wonder that his penalty is judicial execution by the harshest death penalty, i.e. being stoned to death.