Through Hashem's revelation, He expressed His approval of Eliyahu's deeds. Commentators, however, disagree regarding the specific motive of the demonstration:
The revelation at Chorev was a reward for Eliyahu's sanctification of Hashem on Mt. Carmel and his returning the Children of Israel to worship of Hashem.
Why Sinai? Radak claims that Hashem revealed Himself to Eliyahu in exactly the same place that He had revealed Himself to Moshe, identifying Eliyahu's cave with the "crevice in the rock" from which Moshe saw Hashem.
23 The choice serves to heighten the honor bestowed upon Eliyahu as it equates him with Moshe, the greatest of prophets.
Miraculous journey – The miraculous sustenance was also part of Eliyahu's reward. In this, too, there is a comparison to Moshe who survived for forty days and nights without food.
24 "מַה לְּךָ פֹה אֵלִיָּהוּ" – Radak suggests that this question is simply a way of entering into conversation with Eliyahu.
'"קַנֹּא קִנֵּאתִי לַה" – Radak reads Eliyahu's response not as a cry against the Children of Israel, but as a tirade against Izevel and her Ba'al prophets who caused Israel to go astray, destroyed God's altars, and killed the prophets of Hashem.
25 Wind, earthquake and fire – Radak does not explain the individual significance of each of the natural phenomena, nor of the contrast to the "small, still voice," suggesting only that together they were a show of honor to the prophet.
26 It is possible that the strong forces of nature served to herald Hashem's approach, as a trumpet might announce the arrival of a king. The emphasis on the absence of Hashem's presence in these forces might be practical rather than symbolic, a way of informing Eliyahu when it was, or was not, necessary to cover his face.
"מַה לְּךָ פֹה אֵלִיָּהוּ" take two – When rewarding Eliyahu, Hashem did not suffice with a revelation, but asked the prophet to request something of Him. The question "מַה לְּךָ פֹה אֵלִיָּהוּ" is not accusatory in tone, but a straightforward question meaning "what is it that you would like".
27 '"קַנֹּא קִנֵּאתִי לַה" take two – In response to Hashem's question, Eliyahu responds that he desires vengeance.
Appointments: "הַנִּמְלָט מֵחֶרֶב חֲזָאֵל יָמִית יֵהוּא" – Hashem's directives to appoint Chazael, Yehu, and Elisha represent His acquiescence to Eliyahu's request. Through them, there was to be vengeance on all who worshiped the Baal.
28 "וְאֶת אֱלִישָׁע... תִּמְשַׁח לְנָבִיא תַּחְתֶּיךָ" – This position does not see Elisha's appointment as a sign that Eliyahu's tenure as prophet had ended,
29 but rather as a preparation for the future. Hashem recognized that Eliyahu was tiring, and suggested that he train someone to eventually take his place.
Biblical parallels – Our story is not the only one in which Hashem reveals Himself to man as a show of honor. Ramban suggests that Hashem's appearance to Avraham after being circumcised, and to Israel after consecrating the Mishkan, serve the same purpose.
Through the revelation, Hashem renewed the appointment of Eliyahu, encouraging him to continue in his mission.
Sources:Prof. U. Simon
Flight and desire for death – Eliyahu's flight is prompted by Izevel's call to kill him, but his desire to die stems from a feeling of having failed in his prophetic mission.
Miraculous journey – Hashem gives Eliyahu miraculous sustenance to encourage him and prove to him that, contrary to Eliyahu's claim, he is actually "better than his fathers", and, thus, merits a miracle previously merited only by Moshe.
30 '"קַנֹּא קִנֵּאתִי לַה" – In this short speech, Eliyahu explains the reason for his despair and desire to quit: the people have broken Hashem's covenant, ruined His altars and killed His prophets. The initial success of the demonstration at Mt. Carmel was apparently short-lived. Izevel's dismissal of Achav's report regarding the event, and the ease with which she resumes her chase after the prophet, suggests that she was able to reverse any impression left on the king by Eliyahu. Eliyahu knew that it would not take long for the people, too, to resume their old idolatrous ways.
"צֵא וְעָמַדְתָּ בָהָר לִפְנֵי י"י" – Throughout his career, Eliyahu referred to his position as prophet as "one who stands before God."
31 Thus, now, when he no longer wishes to prophesy, Hashem tells him, "צֵא וְעָמַדְתָּ.. לִפְנֵי י"י", come back and once again stand before God, resume your job.
Revelation: wind, earthquake and fire – Prof. Simon suggests that the revelation serves as a second annunciation of the prophet. Hashem is not wiling to accept Eliyahu's resignation, and instead renews his appointment. The forces of nature precede the appearance of Hashem, like runners before a king's a chariot. They appear in full strength before the exposed Eliyahu serving simultaneously to both shock and encourage him with the awesome power of God.
"קוֹל דְּמָמָה דַקָּה" – Hashem Himself appears as a soft rustle, the aftermath of the storm. Prof. Simon does not explain the significance of this choice, but compares the two stages of the revelation here, to that described by Elifaz in Iyyov 4: "וְרוּחַ עַל פָּנַי יַחֲלֹ ... יַעֲמֹד וְלֹא אַכִּיר מַרְאֵהוּ... דְּמָמָה וָקוֹל אֶשְׁמָע".
Biblical parallels: renewal of prophecy – Prof Simon points to Yeshayahu 49:1-6 and Yirmeyahu 15:10-21 as other examples of a "renewal of prophecy". Each of these units contains the same basic components: 1)despair of the prophet and request to die 2) declaration of failure in fulfilling his mission 3) blaming of the nation and 4) renewal of prophecy, via encouragement to the prophet.
32 Eliyahu's case stands out in the dramatic nature of the re-annunciation.
"מַה לְּךָ פֹה אֵלִיָּהוּ" take two – Hashem's question is an invitation to the prophet to re-accept his mission.
"קַנֹּא קִנֵּאתִי לַי"י" take two – Eliyahu responds that Hashem's revelation has not changed his feelings nor his failure in his war against the Baal.
"לֵךְ שׁוּב לְדַרְכְּךָ" – Prof. Simon reads this as Hashem's encouragement to Eliyahu to, nonetheless, resume his position. Hashem follows this statement with a promise of aid,
33 providing Eliyahu with the means to return to his old path.
The appointments – Hashem's response to Eliyahu represents Hashem's total agreement with Eliyahu's position. Hashem, too, thinks that the nation deserves punishment, and issues directives to bring it. However, recognizing that Eliyahu is not up to the task himself, Hashem tells him to appoint assistants to help, much like Hashem had Moshe anoint 70 elders when he reached his breaking point.
34 "וְאֶת אֱלִישָׁע... תִּמְשַׁח לְנָבִיא תַּחְתֶּיךָ" – Prof. Simon suggests that the verse does not mean that Eliyahu is to appoint Elisha in his stead, but rather as his successor.
35 After all, the goal of Hashem's revelation was not to fire Eliyahu, but to re-anoint him.