What is the meaning of "Vayikra El Ish ha'Elokim"?
Rashi: The false Navi called to the true Navi.
Radak citing Targum Yonasan: [The false Navi] had Nevu'ah about the Navi of Hashem.
Radak: The Nevu'ah came to the Navi Hashem. "El ha'Navi Asher Heshivo" means 'to the [true] Navi, that the elder returned him.'
Why was the true Navi punished? The elder lied to him!
Radak (18): He should have investigated to see if he is a true Navi.
Malbim: One must be stringent about every Safek about Hashem's command. If He did not clarify, He wants us to be stringent. This is the Mishmeres - we must guard the Mitzvah with all its conditions, stringencies and fences, and not be lenient. Since "Hashem commanded you", you should not have relied on someone else to overturn it, until Hashem Himself does. This is why we do not believe a Navi who says that Hashem permitted idolatry, even for a Hora'as Sha'ah - because we heard "Anochi" and "Lo Yihyeh Lecha" directly from Hashem.
