"And Samuel said to Saul, “You have done foolishly. You have not kept the command of the LORD your God, with which he commanded you. For then the LORD would have established your kingdom over Israel forever"(1 King 13:13).
Does this affirm open theism? Is Samuel using "possibility" as a real metaphysical predicate of the impact of Saul's behavior on the future? Is he affirming the power of contrary choice? Doesn't this passage imply that the future could have been otherwise, such that the future is not totally determined, necessary or certain?
This passage has nothing to do with modality? Contrary to open theists, who argue that this passage proves that things could have been otherwise,
God has already established in Genesis that the Messiah would be from the tribe of Judah:
"The scepter shall not depart from Judah, nor a lawgiver from between his feet, until Shiloh come; and to him shall the gathering of the people be"(Gen. 49:10).
Saul was of the tribe of Benjamin(1 Sam. 9:1, 4, 16, 21). So Saul's kingdom could not have been established forever. Indeed, this is affirmed in 2 Samuel of David's kingship(2 Sam. 7:1-12). This is appropriate, since David was from the tribe of Judah (Ruth 1:1, 4:17). The most important point is that of Gen. 49:10, where God establishes from the very beginning that the Messiah would be from the tribe of Judah and not that of Benjamin.
It should therefore come as no surprise that God explicitly says that King Saul would be a bad king. With the inevitability of his disobedience comes the inevitability (along with the prophecy concerning the Messiah coming from the tribe of Judah) of the fall of his kingship.
"When Samuel became old, he made his sons judges over Israel. 2 The name of his firstborn son was Joel, and the name of his second, Abijah; they were judges in Beersheba. 3 Yet his sons did not walk in his ways but turned aside after gain. They took bribes and perverted justice.
4 Then all the elders of Israel gathered together and came to Samuel at Ramah 5 and said to him, “Behold, you are old and your sons do not walk in your ways. Now appoint for us a king to judge us like all the nations.” 6 But the thing displeased Samuel when they said, “Give us a king to judge us.” And Samuel prayed to the Lord. 7 And the Lord said to Samuel, “Obey the voice of the people in all that they say to you, for they have not rejected you, but they have rejected me from being king over them. 8 According to all the deeds that they have done, from the day I brought them up out of Egypt even to this day, forsaking me and serving other gods, so they are also doing to you. 9 Now then, obey their voice; only you shall solemnly warn them and show them the ways of the king who shall reign over them.”
Samuel's Warning Against Kings
10 So Samuel told all the words of the Lord to the people who were asking for a king from him. 11 He said, “These will be the ways of the king who will reign over you: he will take your sons and appoint them to his chariots and to be his horsemen and to run before his chariots. 12 And he will appoint for himself commanders of thousands and commanders of fifties, and some to plow his ground and to reap his harvest, and to make his implements of war and the equipment of his chariots. 13 He will take your daughters to be perfumers and cooks and bakers. 14 He will take the best of your fields and vineyards and olive orchards and give them to his servants. 15 He will take the tenth of your grain and of your vineyards and give it to his officers and to his servants. 16 He will take your male servants and female servants and the best of your young men[a] and your donkeys, and put them to his work. 17 He will take the tenth of your flocks, and you shall be his slaves. 18 And in that day you will cry out because of your king, whom you have chosen for yourselves, but the Lord will not answer you in that day""(1 Sam. 8:1-18).
So this is precisely in accordance with God's plan. The Davidic, kingly Messiah would come from the tribe of Judah, as God had declared, and King Saul would disobey God and incur His wrath, which would entail the rejection of his kingship. God's giving of King Saul to Israel as a king was His judgment on them for demanding a king in God's stead, as 1 Samuel 8:1-13 explicitly says. As for 1 Sam. 13:13 - Samuel is simply speaking in ordinary, colloquial terms. Obedience entails God's blessing and disobedience incurs God's wrath. As a Calvinist, I can say, in colloquial language, that if someone lives in a sin, they will be condemned, and that if they had not lived in sin but had had faith in Christ instead, they would not have been condemned. But this does not imply a strict, philosophically sophisticated modal possibilism. I can still coherently hold that it was not actually possible for them to have lived in such a way.