Shared ground
Jeremiah 28:8–9 treats prophecy as a public claim about reality, not just an inspiring message. Jeremiah points to an established pattern: earlier prophets often announced trouble—war, disaster, and disease—against “many countries” and even “great kingdoms” (v.8). Then he names a practical rule for a different kind of message: when a prophet predicts “peace,” that claim is confirmed only when it actually comes true (v.9). The passage assumes Yahweh is able to authenticate a messenger in history by making the prophet’s “word” happen.
Where interpretation differs
Some readers take v.8 as saying harsh prophecy is the normal, default posture of true prophets, so reassuring messages should be viewed with special suspicion. Others read v.8 more modestly: Jeremiah is describing a common historical pattern (many prophets did warn of judgment), not laying down a timeless rule that true prophecy is usually negative.
Some also differ on what “peace” covers. It can be read narrowly as absence of war, or more broadly as national well-being and stability (the kind of turnaround Hananiah promised in Jeremiah 28).
Why the disagreement exists
The wording in v.8 sounds broad (“the prophets… of old”), but it is also a rhetorical move in a specific dispute. And “peace” is a flexible Bible term, so interpreters decide whether Jeremiah means a specific political outcome (security from Babylon) or a wider picture of flourishing.
What this passage clearly contributes
Explicit claims: (1) Earlier prophets often spoke against nations and kingdoms with warnings of war/disaster/disease. (2) A prophet can prophesy “peace.” (3) A peace prediction is recognized as truly from Yahweh when the spoken “word” happens.
Reasonable inference: the passage prioritizes verifiable fulfillment as a key test for reassuring predictions, especially when competing voices are claiming divine authority in the same crisis (compare Deuteronomy 18:21).