Sent to Nineveh to preach repentance, Jonah saw first hand how God could forgive even foreigners.
The message was also intended for King Jeroboam II of Israel, who spoke with the king, and (probably) King Uzziah of Judah: it’s never too late to repent and turn back to God. Nineveh did repent, and God held back judgement:
Jonah 3:10 When God saw what they had done and how they had put a stop to their evil ways, he changed his mind and did not carry out the destruction he had threatened.
Amos’ message came around the same time as Jonah’s. It was harsher and more pleading. It took the same form for each place he spoke about:
The people of [a place] have sinned again and again,
and I will not let them go unpunished!
But God was still wanting His people to turn back to Him:
Amos 5:6 Come back to the Lord and live!
Otherwise, he will roar through Israel like a fire,
devouring you completely.