Q. Does God wait for a people to surpass a certain point before He brings judgement on them? Do we have the means to know what that point is? I ask because in Genesis, when Abram falls in a deep trance, God says that the iniquity of the Amorites hasn’t yet reached its full extent. We learn that the Amorites were later destroyed, during Moses’s time. The timeline seems to fall in place with the conversation God had earlier on with Abram. However, Moses says in Deuteronomy that God made Sihon king of the Amorites stubborn and obstinate. Why wasn’t Sihon given more opportunities perhaps to repent, as the arrogant Pharaoh of Egypt was given before God hardened his heart? In light of all this, how do we take part in God’s story though history, and are some people predestined for destruction regardless of how thing turn out?
Let me start with the later part of your question and work my way back to the beginning of it. While Moses does say that God made Sihon’s heart stubborn, I think we actually should see the way that God treated Sihon as similar to the way that God treated Pharaoh. First each ruler chose to disregard God’s supreme power and authority, despite warnings, and then God confirmed the ruler in that choice.
We learn from other Old Testament narratives that the peoples living in the area where the Israelites ultimately settled were aware of what God had done to free them from slavery in Egypt. Late in the time of the judges, for example (under Samuel, the last judge), the Philistines were fighting against the Israelites and they learned that the ark of the covenant had been brought onto the battle line. They recognized the ark as an embodiment of the presence of God, and they cried out, “Who will deliver us from the hand of these mighty gods? They are the gods who struck the Egyptians with all kinds of plagues!” The Philistines fought extra hard because of this, and while they won the battle and captured the ark, they were then struck by plagues themselves and had to return the ark in order to be delivered.
Similarly, when Joshua sent spies to explore the land of Canaan, Rahab told them, “All who live in this country are melting in fear because of you. We have heard how the Lord dried up the water of the Red Sea for you when you came out of Egypt, and what you did to Sihon and Og, the two kings of the Amorites east of the Jordan.” So news of God’s earlier victories spread to the surrounding peoples, and this can be considered a warning that they should have heeded. Rahab herself expressed faith in the true God and helped the Israelites, and so she was spared with her family. I think we can conclude that Sihon should have known enough, from what he must have heard about God striking the Egyptians with plagues and drying up the Red Sea so the Israelites could escape Pharaoh’s army, to allow them to pass through his territory, which was their only request. But instead of allowing them to go by peacefully, he attacked them, and in the battle that followed, he and his army were destroyed. We don’t know exactly where his own choices ended and God’s confirmation of those choices began, but in Deuteronomy Moses is describing the latter part of that process.
This may point to at least part of the answer to your original question. If a people and its leaders are no longer considering the warnings that God has given them—particularly, in the case of modern societies, in the biblical record of God’s dealing with humanity in former times—then we should indeed be concerned that they may have passed an ominous point. This is true at least descriptively: If people are no longer heeding God’s warnings, how can they be expected to change course?
But I would hesitate to say this prescriptively, that is, “Once we see a people reach this or that particular stage, we know that they’ve passed the point of no return.” To answer the first part of your question directly, I don’t believe we have the means to know that what point is. So we should always pray and work for God’s standards of justice and compassion to be honored in our societies. Even in a situation where we may think that things have gotten so bad there’s no way we our society can escape divine judgment, we can still respond with repentance, prayer, and advocacy. We can say, as the king of Nineveh did even after Jonah told him that his city was going to be destroyed, “Who knows? God may yet relent and with compassion turn from his fierce anger so that we will not perish.” In short, I don’t believe that any particular people is predestined for destruction regardless of how thing turn out.