Answer:
The idea of divine determinism comes from the postulation that the divine is omniscient (all-knowing). If the divine is omniscient, then the divine must know everything.
If the divine knows everything, then the divine knows everything that has happened, is now happening, and will happen.
If the divine know everything that will happen, then all events must happen the way that the divine foreknows them. If so, we arrive at the idea of divine determinism, which is the idea that the divine determines every future event.
The problem is that this allows no room for human free will. Why? If everything is foreknown, then there is no contingency. If there is no contingency, then free will is an illusion.