Why do all the things which are forbidden by the Torah have corresponding similar things which are permitted?
(12,3) “And you shall tear down their altars and smash their monuments and burn their Asherim with fire, and you shall cut down the graven images of their gods and destroy their names from that place. You shall not do so to Hashem your G-d.”
The Sifri asks on our posuk which says “you shall tear down their altars…you shall not do so to Hashem your G-d” - would anyone think that Yisrael might tear down the altars! (So why would we need to be told not to do so?) Rather, the Torah is warning us not to erase the name of Hashem.
But surely the question that the Sifri asked still stands - would anyone think that Yisrael would erase Hashem’s name?!
But we can explain the Sifri according to the gemora Chullin 109b which teaches that for everything that the Torah has forbidden to us, there exists something equivalent which is permitted. (One of the examples that the gemora gives is that we are forbidden to eat pig meat, but permitted to eat the brain of the Shibbuta fish which has an similar taste). And the reason why Hashem made it so is because Chazal teach that a person should not say that he does not want to eat pig meat, but rather he should say that he really wants to eat it but the Torah forbids it.
Therefore Hashem made equivalent permissible things for all the things which He prohibited in order to allow us to experience the forbidden thing in a permissible fashion, and even so restrain ourselves from that which is prohibited. Because if we could never experience the forbidden thing a person would stay away from it simply because it is loathsome to him, even though he has never tried it.
Thus the Sifri was asking how is it possible that there is a prohibition to tear down the altar - we do not find anywhere something which is permitted which corresponds to this! Therefore it answers that the Torah is warning us here not to erase Hashem’s name, and for this prohibition we do find a corresponding permissible act - the erasing of Hashem’s name for the sake of a Sotah.