Why should one not listen to the Navi who substantiates his prophecy with a miracle?
Ramban: Because Hashem, whom we saw firsthand at Yetzi'as Mitzrayim is Master of the world, 1 and that there is no other god like Him, commanded us 2 at Ma'amad Har Sinai, not to worship any deity other than Himself. 3
Seforno: Because, since part of his prophesy is meant to is to take you away from Hashem, it is clear that there is no truth in his worashem, and his miracle was based on witchcraft or something similar, and not on prophecy at all. 4
What is the objective of the test?
Seforno: To see if you love Hashem, because, if you do, you will consider the prophet who is trying to veer you away from Hashem as an enemy, and not even attempt to see whether there is any truth in his words - thereby sparking off a reciprocal reaction from Hashem, who showers the world with goodness.
Oznayim la'Torah: To demonstrate to the people that they do not love Hashem 'be'Chol Levavam u've'Chol Nafsham'. 1
What is the point of the test, seeing as Hashem knows what is in is person's heart?
Ramban (in Vayeira, Bereishis, 22:1): It is to convert the potential into action - in order to reward the Tzadik 1 for passing the test.
Ramban: In fact, Hashem only tests Tzadikim tests are based on His love of the Tzadikim because He loves them, as the Pasuk writes in Tehilim, 11:5 "va'Hashem Tzadik Yivchan".
Why does the Torah insert the (otherwise superfluous) words "be'Chol Levavchem u've'Chol Nafsh'chem"?
Oznayim la'Torah: The Torah is speaking about a case where the Navi instructs the people to serve Avodah Zarah in order to bring rain - and they follow his instructions and it does indeed rain. Based on the Pesukim in 'Vehayah Im Shamo'a', Hashem sends rain if the people 'love Him with all their heart and with all their Soul'. Consequently, the fact that it requires the Navi Sheker to bring rain is proof that they do not.
