Well, determinism has been discussed a lot here; some of us are more deterministic than others, and there are different degrees of the concept, just as there are different degrees of the concept of free will.
My own position on this isn’t easy to summarize, but I develop it in Section Three of SttH, the links to each chapter of which can be found here on the forum.
Like C. S. Lewis, whom you quote in your sig, I’m not particularly deterministic. But I also accept Lewis’ Boethian notion of transcendent omniscience (and omnipresence). It’s important, when working in a supernaturalistic theism, not to accidentally go back to putting God’s fundamental existence within the timestream of Nature; and that’s what the idea of the future being ‘locked’ in or ‘determined’ by virtue of God’s omniscience amounts to. In other words, because God foresees X, God must make X happen (or something superior to God must make X happen???) to ensure X.
Boethian omniscience of the sort accepted by Lewis (in Miracles: A Preliminary Study 2nd edition for example), doesn’t negate the free will of the creature or reduce it to merely being a capability for behavior. God sees me freely acting (contributing active choices to the set of natural events) at points X, Y and Z, before, during, and after my present moment. (Of course I am also reacting and counterreacting automatically to stimuli beyond human counting at each point; I’m not free from being a natural creature, too.) From my perspective God ‘looks ahead’ to see what I will do, but God presently sees it all at once.
This does however make for some difficulties in communicating to us what is seen; and then sometimes God promises to bring about certain results, but He does so in view of what He can already see occurring thanks to the contributions of various rational creations along the timeline – and in view of God’s own contributions to the whole timeline up to and including (and after!) the point in question.
All of which is somewhat different than (but also related to) the question of how God can make room for even the limited, derivative freedom of creaturely rational action (and not merely creaturely capabilities). My solution to that ancient riddle involves the ongoing self-sacrifice of the self-existent, self-begotten God; following out some hints Lewis (and MacDonald before him) never quite worked out systematically. I was pleased to see that the results seem to mirror the very odd characteristics of quantum percolation and zero-point energy, though!