The beginning of the reign of Jehoiakim was in 609 BC. The order is not chronological in Jeremiah, the date here sets this utterance in chapter 26, three years before the last, so this prophecy is about nineteen years after he was first commissioned to prophesy.
God knows every future event, and he knows those whom he will cause to believe and turn to him, and he knows those who will not turn. The fact that he knows that some will not turn does not mean that God is not sincere in offering mercy. The promise opens up a very real possibility to the sinner to be forgiven, and God by this offer puts the responsibility firmly in our lap. Will we respond? Let no one say God’s offer of mercy is not genuine?
No preacher can be embarrassed to speak about sin, or to oppose head-on the waywardness of man. There are many encouraging and gracious things that the gospel preacher needs to say, but there are also some very direct things, and he must not flinch from saying them. He is God’s messenger to the world, and he is not being faithful if he tones down the message or alters it to take away from its convicting power.