In regard to the ministry of Jeremiah there are some historical questions I wonder about?
In his early ministry he spoke of a threat from the north (4:6, 15-16; 6:22). Since Babylon arose decades later, what was he talking about?
Did he support the reforms of King Josiah? There is no explicit text about this. The reforms did not mitigate the condemnation of Judah in 6:27-29.
Like I said, I wonder about these questions. I don’t know the answers.
A widely accepted answer to the first question is that he was talking about a Scythian invasion. These were tribes whose homeland was north of Black Sea. They apparently were very war-like and launched military campaigns all over Asia. There is even evidence that they fought the Chinese. (Also there is evidence that they were stoners (see here).
The Greek chronicler, Herodotus, says that in the 6th century they attacked south and came to the borders of Egypt. They turned back after the Egyptians gave them gifts. They ransacked Ashkelon, the Philistine city in Gaza. So, assuming Herodotus knew something about a real campaign, the Scythian invasion missed Judah and stayed along the coast. But it may be that Jeremiah (and Zephaniah) used the threat of the Scythians to warn Judah of approaching judgment.
If so, Jeremiah may have been discredited in his early days, since his prophecy appeared false. This may account for his apparent despair and a period of silence. Only years later, when Babylon arose in the north, did he rehabilitate himself by claiming that the revelation he had received had been about Babylon all along.
About his connection to Josiah’s reforms, we know that people from his home town of Anathoth were out to get him (11:21). One theory about this is that Josiah’s reforms, which centralized worship in Jerusalem, undercut the livelihoods of Levites who tended other shrines. This would have included Jeremiah’s priestly relatives from Anathoth. So, if Jeremiah had supported Josiah’s reforms, he would have aroused the anger of his own family. But some of his family also seem to have been royal officials who supported and even instigated the reforms.
It is complicated, but Jeremiah might have supported the reforms and then become disillusioned that they dealt more with the surface than with the heart.
The reason we don’t know more, I think, is because Jeremiah’s scribe, Baruch, who probably put our book of Jeremiah together, became involved later in the prophet’s life and had a fuzzy picture of his early career. Also his purpose was not to give us accurate biographical information. He gives us some, but only by accident on his way to presenting a theological account.