Open theism, in a nutshell, asserts that the future free choices of agents endowed with free will are unknowable, and therefore that God, though omniscient, is not capable of knowing in advance what we will freely choose in any particular future situation because future free choices are unknowable things, as I just said.
“Open theism” is an attempt to explain the foreknowledge of God in relationship to the free will of man. The argument of open theism is essentially this: human beings are truly free; if God absolutely knew the future, human beings could not truly be free. Therefore, God does not know absolutely everything about the future.
Open theism holds that the future is not knowable. Therefore, God knows everything that can be known, but He does not know the future. Open theism bases these beliefs on Scripture passages which describe God “changing His mind” or “being surprised” or “seeming to gain knowledge.”
For example, in the book of Job it seems that God didn’t know what the outcome would be and so he changes his mind. Through his prophet, God had told Nineveh He would destroy the city in forty days (Jonah 3:4). However, Nineveh repented of their sin (verses 5–9). In response to the Assyrians’ repentance, God relented:
When God saw what they did, how they turned from their evil way, God relented of the disaster that he had said he would do to them, and he did not do it. (Jonah 3:10).
Prior to the flood, it seems like the wickedness of humanity caught God out and he felt regret. His disappointment is used to suggest that God was not aware of how things would turn out:
And the Lord regretted that he had made man on the earth, and it grieved him to his heart. (Genesis 6:6).
Another instance where it looks like God didn’t know how things would turn out is when Abraham was prepared to sacrifice his only son, Isaac. God had to test Abraham’s faith:
He said, “Do not lay your hand on the boy or do anything to him, for now I know that you fear God, seeing you have not withheld your son, your only son, from me.” (Genesis 22:12).
After the Israelites offended God by making an image of a golden calf, God’s anger burned against them to the extent he was going to destroy them. But God changed his mind:
And the Lord relented from the disaster that he had spoken of bringing on his people. (Exodus 32:14).
There may be other Bible verses that the proponents of Open Theism use to support their views but I’ve run out of time for today and must sign off. You have not asked for a Christian rebuttal so I will simply refer you to a section in the Wikipedia article which criticises open theism and presents a few verses that support the immutability of God: https://en.wikipedia.org/wiki/Open_theism#Criticism
As some have implied erroneously, it is not a question of what God is capable of, he is capable of anything and everything; it is what has he reveal concerning what he did do. Open Theology is simply asserted that God has chosen to operate in real time rather than predestinate everything. There are three views to history. It is all predetermined, Calvinism, Arminianism, Sovereign Grace, Presbyterianism; God's forknowledge is due to him having predestinated all things. Or, history is not predestinated but just exist and God looks at the past, present, and future all at once to see what happened, neo-Arminianism (my designation); God's foreknowledge is due to him seeing what happened as it just happened to happen apart from him.
Then there is Open Theology. History is running in real time under the guidance and control of God keeping all within his plan, leading all to his predetermined conclusion. God's foreknowledge would be of those things that he has predetermined from the foundation or judgments made in the course of history. his prophecies are just telling us what he is going to bring about as he guides and directs history. Many of which are pronounce as judgments upon men and nations for their wickedness and were not from the foundation or beginning.
Isaiah 55:11 KJV So shall my word be that goeth forth out of my mouth: it shall not return unto me void, but it shall accomplish that which I please, and it shall prosper in the thing whereto I sent it.
The prophecies are not predictions or prognostications of what he sees in the future but foretelling what God will bring to pass directing the course of history in real time.
Predestination theologies strongly premise their view on the supposition that if all is not predetermined by God then he would not be sovereign. It would be hard to find a more unreasoned, unbiblical, assertion. God is the creator and sustainer of all things; how can he not be sovereign in whatever he does or chooses to do. Regardless of what he elects to do it would be his sovereign will. He would be just as sovereign if after creation he sat back in an easy chair to watch what happened. To insist on anything else would be to limit God.
Psalms 78:41 KJV Yea, they turned back and tempted God, and limited the Holy One of Israel.
This is a major error in the premise of predestination; God does not like to be described as limited.
Even those advocating predestination admit that their doctrine contradicts many passages of Scripture. Any time a teaching conflicts with the obvious meaning of a passage of Scripture it is a clear sign that some part of it is in error. The Bible does not contradict itself.
The idea that God has to look down upon history to see what happen is the most lackluster God limiting view of these theologies. In Open Theology, having history flow in real-time, raises the Lord to heights the others cannot even imagine. Working in real-time, knowing every thought and intent of men as they have them in the moment, knowing all things as they happen, having them in control, guided, and kept within his plan, God turns everything, all Satan’s devises, his and man’s evil intents, to his own purpose. He judges and passes judgment on all and records all in his books, all in real-time. It is mind-boggling; man cannot fathom the depths of the wisdom and power of the Almighty.
Open Theology, as I see it, is the only one that fits the Scriptures without causing contradictions and confusion and otherwise limiting God.