Sforno on Deuteronomy 13:4/ספורנו על דברים י״ג:ד׳

1לא תשמע אל הנביא ההוא, do not even turn to listen to find out if his words have even partial substance, for there can be no question that the man is full of lies and tells you things he has made up, all for an evil purpose; כי דבר סרה, for what he portrays as a miracle is nothing but sleight of hand or forgery of some other kind.

2או אל חולם החלום ההוא, to see if even part of what he dreamt reflected some truth of what was going to happen; you may be sure that the dreamers did not actually dream any part of what he claimed to have dreamt, but he invented all of it in order to seduce you;

3כי מנסה ה' אלוקיכם אתכם, by referring with hostility to a person who spoke out against G’d, you will be proven as truly loving G’d.

4לדעת, once your previously theoretical faith in G’d has been so tested, you will become truly tested in your intimate conviction of G’d’s goodness which becomes an unshakable enduring conviction. Someone who believes truly in the goodness of the Lord cannot help but hate anyone who denies G’d’s goodness, just as if you love a human being truly you cannot help hating a person who hates the person whom you truly love.