וַיִּגַּ֨שׁ אֵלִיָּ֜הוּ אֶל־כָּל־הָעָ֗ם וַיֹּ֙אמֶר֙ עַד־מָתַ֞י אַתֶּ֣ם פֹּסְחִים֮ עַל־שְׁתֵּ֣י הַסְּעִפִּים֒ אִם־יְהוָ֤ה הָֽאֱלֹהִים֙ לְכ֣וּ אַחֲרָ֔יו וְאִם־הַבַּ֖עַל לְכ֣וּ אַחֲרָ֑יו וְלֹֽא־עָנ֥וּ הָעָ֛ם אֹת֖וֹ דָּבָֽר׃
Elia si accostò a tutto il popolo e disse: «Fino a quando zoppicherete fra due opinioni? Se il Signore è Dio, seguitelo; ma se è Baal, seguitelo». E il popolo non gli rispose una parola.
Akeidat Yitzchak
What our Parshah deals with is something else. When two men, each claiming to be a prophet, both adhering to Torah law, neither of whom having established a reputation as a prophet, make conflicting predictions, then a test is needed to determine who is the true and who is the false prophet. In such an event, the fulfilment of the prophecy of one of them regardless of whether the prediction had been a promise or a threat of a calamity, serves as the yardstick that enables us to separate truth from falsehood. To make the occurrence of the "good" prophecy proof, and the non occurrence of the "bad" prophecy a yardstick for truth, would result in a chillul hashem, desecration of the Lord's name. The case of Elijah on Mount Carmel is a good illustration. Since the prophets competed in drawing fire down from Heaven, the successful one would become legitimised At that time it was imperative to discredit the priests and prophets of the Baal. (compare Kings I chapter 18) In our Parshah, the paragraph commencing with the words "when you will say in your heart, how will we know?", (18,21) must not be understood as introducing a new subject matter dealing with every kind of prophet, but only as referring to something that had already been discussed. First, we must be assured by G'd that we will have prophets in our midst. Then the Torah tells us the task of such prophets. Should someone fail to obey the prophet, by claiming to be a prophet also, such a person is to be executed. How could we know which of the competing prophets to execute as an impostor? Whenever the prophecy fails to be fulfilled down to the last detail. Since there are no other criteria to establish credibility, a miracle is not only in order, but becomes mandatory. The Torah instructs us not to be afraid to execute such an impostor as there is no distinction between intentional and unintentional commission of such a crime. It can only have been committed intentionally. (verse 20-22) "The name of other deities should not be heard from your mouth." This makes the point that unless the prophecy was made in the name of the living G'd, waiting for the result of any test is quite unnecessary. The peculiar expression "that is the word G'd has not spoken," needs to be identified, puts the onus on the liar to prove something.
Kuzari
14. Al Khazari: A wide difference should be made between the party of Jeroboam and that of Ahab. Those who worship Baal are idolators in every respect. In reference to this Elijah said: 'If the Lord be God, follow Him; but if Baal, follow him' (I Kings 18:21). For this reason the Sages are in a dilemma as to how Josaphat could partake of Ahab's food. They have no such doubts concerning Jeroboam. Elijah's protest had no reference to the worship of the calves, since he said: 'I have been very jealous for the Lord, the God of Israel' (I Kings 19:10). The party of Jeroboam considered itself belonging to the Lord, the God of Israel,' also all their actions, their prophets were the prophets of God, whilst the prophets of Ahab were Baal's prophets. God appointed Jehu to destroy the works of Ahab. He proceeded with much zeal and cunning, saying: 'Ahab served Baal a little, Jehu will serve him much' (II Kings 10:18). He destroyed all vestiges of Baal, indeed, but did not touch the calves. The worshippers of the first calf, the party of Jeroboam, and the worshippers of the heights and the image of Micah had no other idea than that they were serving the God of Israel, though in the way they did it they were disobedient and deserved death. This is as if a man marries his sister either under compulsion or from lust, and yet observes the marriage regulations as commanded by God. Or if one would eat swine's flesh, but carefully observe the rules concerning slaughtering, blood and ritual.