If we live in a fully deterministic universe, a concept theologians like Calvin, and physicists like Gerard t’Hooft have given at least some credence too, then the clear implication is that free will cannot exist; at least not if the complex web of causality that moves the world, is temporally linear.
It appears to be generally accepted though, that we live in a probabilistic universe, wherein a maelstrom of random occurrences (de)cohere into a plausibly determinist macroscopic narrative. So the sense we have of exercising agency in our own lives, is no more (or less) illusory than any other facet of our experience.
Perhaps we can never fully understand the infinitely complex confluence of forces and influences which drive and inform not only our choices, but our will itself. Sometimes anyway, it pays to keep things simple; to trust in God, and seek His will for us.