ירמיהו, פרק כ״ח, פסוק ט׳

A true prophet must be able to stand behind his messages, particularly when they bring promises of peace and prosperity. There is a fundamental difference between a prophecy of doom and a prophecy of comfort. If a prophet foretells disaster, the decree might be canceled if the people repent, and this reversal does not invalidate the prophet. However, a positive promise must come true to maintain the prophet's credibility [חומת אנך].

When an individual arrives contradicting the warnings of destruction and instead promises a positive future marked by a specific event [מצודת דוד, ביאור שטיינזלץ], the people cannot believe him immediately simply because his message is pleasant to hear [מצודת דוד]. In some cases, the primary purpose of delivering a positive prophecy is to firmly establish the individual's status as a true prophet [מלבי״ם]. Therefore, the ultimate proof of authenticity arrives only when the promised event actually takes place. At that moment, it becomes clear that God truly sent him. If the positive prophecy fails to materialize, the prophet is immediately proven false [ביאור שטיינזלץ]. Yet, the promise does not necessarily have to be fulfilled in its absolute entirety. If even a portion of the prophet's words comes true, it is sufficient to prove his authenticity [חומת אנך].

Jeremiah directs these principles toward Hananiah to clarify his current standing. Because Hananiah is delivering a positive prophecy, there is no immediate reason to believe him over Jeremiah, and his status as a prophet remains unestablished. He must wait until his promises become reality. Only through their actual fulfillment will it be definitively proven that God indeed sent him [מצודת דוד, מלבי״ם].