Harry Potter and the Prisoner of Azkaban has a bizarre time travel sequence that also reminds me of the Reformed tension between predestination and free will. Harry and Hermione are lost in the woods. These scary Dementors arise and try to kiss out Harry Potter's soul. Harry tries to do his Patronus charm but can't. Suddenly a stag Patronus comes and saves Harry.
Not too much later, Harry and Hermione go back in time and wait for their past selves to get to the same point. Harry sees the Dementors preying on himself on the other side of a lake and wonders why that stag Patronus has not come. Then he realizes that he sent the Patronus to himself! He then sends it and saves himself.
In JK Rowling's world, Harry was destined to send his Patronus to save himself. It was already planned. At the same time, Harry had to make the decision to send his Patronus. The whole of history was already planned, but Harry is still responsible for his decisions. He still has a free will.
It will always be a mystery, but the Bible teaches it and I believe it. Acts 13:48, "And when the Gentiles heard this, they began rejoicing and glorifying the word of the Lord, and as many as were appointed to eternal life believed."