Predestination may threaten free will because:

a. If God has foreknowledge of every event, this includes our actions, and that means that we can't do anything other than what we do.
b. If God has foreknowledge of every event, this includes our actions, and that means that we are being enslaved or brainwashed.
c. If no event is caused by any other event, this won't include our actions, meaning we won't be able to predict our own actions.
d. If no event is caused by any other event, this will include our actions, meaning we are enslaved or brainwashed.