Listening to today's first reading is to know that a prophet's call is almost always the last element inserted into our biblical collection of the prophet's oracles. Only after a lifetime of delivering God's word does the prophet (or the prophet's disciples) receive some insight into what he or she was actually called to do.
While the prophets are proclaiming that word, most have just small, intermittent insights into how their specific word fits into God's overall plan of salvation history. They're concerned with the here and now of their daily lives, and how their God-formed consciences force them to react to the people and events they encounter. Though we see the big picture, most of them didn't.
Neither are prophets normally in the business of predicting the future. Their ministry revolves around confronting, not predicting. When they do mention the future, it is -- as the late German professor of Old Testament Hans Walter Wolff always taught -- "to provide us with the future implications of our present actions."
In today's pericope, Jeremiah, at the end of his life, is reflecting on the fact that he was prepared for conflict. Though Yahweh doesn't pull any punches about the opposition the prophet will encounter, Yahweh also assures him that he'll eventually be taken care of: "Be not crushed on their account, as though I would leave you crushed before them; for it is I this day who have made you a fortified city, a pillar of iron, a wall of brass, against the whole land."