The Torah in Parashat Shoftim (Debarim 18:15) commands that once a prophet establishes a reputation as an authentic prophet, “Elav Tishme’un” – we are obliged to obey all his instructions. This requirement applies even if a prophet instructs doing something that contravenes Torah law, as long as he does not intend to permanently uproot the law. If he instructs suspending a rule of the Torah as a temporary measure, then we must obey him, because he has already established himself as an authentic prophet. The only exception is if he instructs worshipping a foreign deity; in such a case, we must disobey the prophet.
The Sefer Ha’hinuch explains that we are commanded to obey the words of a prophet because a person achieves prophecy only after he reaches outstanding spiritual heights, to the point where he can be described as having “his feet on the ground but his head reaches the heavens.” A person who has attained such a level can be trusted to understand things on a much deeper level than the rest of us. Challenging a prophet, the Sefer Ha’hinuch writes, is a “Ta’ut Gamur” – “complete mistake,” and so the Torah commands obeying the prophet’s words.
This Misva applies during the times when there are prophets, and is binding upon both men and women. One who disobeys a confirmed prophet transgresses this prohibition and is liable to Mita Bi’ydeh Shamayim (death at the hands of G-d, as opposed to execution by the court). This applies even to the prophet himself – if he disobeys the prophetic message he received, then he is liable to Mita Bi’ydeh Shamayim.
A prophet is liable to Mita Bi’ydeh Shamayim also in a case of “Kobesh Nebu’ato” – if he “withholds” his prophecy, refraining from communicating it to the people as G-d had commanded. In light of this rule, the Rabbis address the question of how the prophet Yona ran away after receiving a prophecy to be conveyed to the people of the city of Nineveh. How could Yona, a righteous prophet, violate this grave Biblical prohibition by refusing to deliver the prophecy he received?
The Radbaz (Rav David Ben Zimra, Egypt, 1479-1573), in one of his responsa (2:842), suggests answering this question based on a careful reading of the story of Yona. When G-d first speaks to Yona, He instructs him to go to Nineveh and to “call out to it” (Yona 1:2), without specifying any particular message. Since Yona did not receive a command to convey a specific prophecy, he felt that this did not fall under the prohibition of “Kobesh Nebu’ato.” The second time G-d speaks to Yona, however, He instructs him to convey to Nineveh “Ha’keri’a Asher Anochi Dober Elecha” – the message that G-d would communicate to him. Here, Yona was commanded to deliver a specific prophetic message, and so he was required to journey to Nineveh and convey the prophecy he received.
The Radbaz also advances a second theory, suggesting that the prohibition of “Kobesh Nebu’ato” applies only once the prophet accepts the role as prophet. At that point, a prophet who withholds his prophecy violates the prohibition. But if he did not accept the job, then he is not required to convey the prophecy. The Radbaz notes that this answers the question of how Moshe Rabbenu initially refused to return to Egypt and deliver G-d’s message, as G-d had commanded at the burning bush. Seemingly, by refusing to speak to Pharaoh and to Beneh Yisrael, Moshe was in violation of the prohibition of “Kobesh Nebu’ato.” According to this theory, however, Moshe was not bound by this law before accepting the job of prophet. He initially declined, until finally G-d persuaded him to accept the mission, at which point he conveyed the prophetic message which he was instructed to convey.