This is true. But knowing the future without error means that the future is already determined. Your choices, if God knows the future exhaustively, are
1. God determines the future
2. Someone else determines the future.
That someone else must be someone in existence at the time all future events became knowable to God. That would not include you and me, even for the things we think we are responsible for determining, since we didn't exist back then, when God began to know our future.
But think about it. If there is some other being who was determining every future event back when God was beginning to know those future events, isn't that being greater than God? Or since God is the greatest being in existence, isn't that being truly God? In which case, God really is determine all future events, because He knows what they will be and because there isn't anyone greater than God.
"With great certainty" is not the same as without error. But what you've described is something that has been determined by someone else (someone besides yourself in this), so your example fits with either 1 or 2 above.
Meaning that He will determine the fulfillment of that promise. And He foreknows because He has predetermined that fulfillment. That's #1 above.
Then God doesn't know that you, personally, will meet the prerequisite conditions before you exist, right? This is Open Theism. If not, then God is predetermining everything.
Yes. But God doesn't know who that will be until they exist and either meet or don't meet the prerequisites. Thus, God's foreknowledge is not exhaustive, because He hasn't predetermined everything.
If it is true, then why the "but?"
Knowing the future does not mean that the future is already determined. God himself makes allowances for people to change their mind. Thus every time we change our mind and take another course of action, our future changes. Of course, God is foreknowingly aware of that already, but his foreknowledge is not determining the future. He is aware of the future.
Of course, there are future events that God has already let us know that will happen based on He doing his will without regard to human input.
For instance, I Thessalonians 4:
13 But I would not have you to be ignorant, brethren, concerning them which are asleep, that ye sorrow not, even as others which have no hope.
14 For if we believe that Jesus died and rose again, even so them also which sleep in Jesus will God bring with him.
15 For this we say unto you by the word of the Lord, that we which are alive and remain unto the coming of the Lord shall not prevent them which are asleep.
16 For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first:
17 Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord.
There is nothing we can do to influence that event. Since we are Christians, we will be one of those who will be raised or caught in the air.
Foreknowledge does not imply determination
Those with freedom of will make decisions that determine their own future and those decisions may or may not influence the future of others. That is why God tells us to Love God and love our neighbors. We determine our relationship with God and with others
Great certainty is not absolute.
However, I can say with absolute certainly that I Thessalonians 4:13-17 is an absolute certainty. Not because I will determine it, but because God decided to do that.
God foreknew that I would chose to believe before the foundations of the world. That is how He could chose me before the foundation of the world.
Ephesians 1:4 According as he hath chosen us in him before the foundation of the world, that we should be holy and without blame before him in love:
He foreknew, so he chose those who He foreknew would chose to believe.
He did not determine whether we would believe or not. We determined to believe, but God foresaw that
God's foreknowledge is exhaustive.
He does not have to predetermine anything to have complete foreknowledge.
Foreknowledge does not imply, in any way, predetermination.