I don’t believe in predestination, but for someone who does I assume the answer to your queston would be Romans 9:15, “For he saith to Moses: I will have mercy on whom I will have mercy; and I will shew mercy to whom I will shew mercy.”
So you don’t believe that God knows the future?
Its really hard for us to understand that the concept of time is temporal...
There’s another way of looking at predestination besides the usual *God picking some to save and the rest to send to hell.
Scripture tells us that those God foreknew, He predestined to be saved.
IOW, He knew before hand who would accept His gift of salvation and then determined that those whoso responded would be given eternal life and He guaranteed it by predestining them to be saved (not go to hell) as opposed to choosing by some fathomless criteria, to save some and damn the rest.
I hope that explained it because it’s another meaning to those very same words and it’s difficult to say*He predestined some to be saved* without people thinking that it means picking and choosing for people instead of giving them the choice, which Scripture is pretty clear he does.