If every cause has only one potential effect, then determinism is true
In this case, if the universe were simulated 10,000 times, everything would happen in the exact same way every time, because there is no uncertainty in the system, even if the system is very complex
This would mean that having consciousness is more like being a "spectator" rather than an "agent", because there would be no real agency. Instead, every thought/action would just be the unfolding of something which was mathematically certain from the very beginning of the universe
This has troubling implications if one believes in God, because it would mean that nobility/wickedness are totally out of the control of whatever subjects happen to embody those characteristics. It would also mean that whoever created the initial conditions would be entirely responsible for every action that resulted afterwards, and that every action was therefore in the direct will of that creator, down to the meticulous details of events, including outrageous suffering. Additionally, every conscious subject (a spectator having a subjective experience of consciousness) in that creation would be entirely designed and controlled by that creator, for their entire lifetime
If any mechanism in the universe introduces probability, then determinism is false
Suppose that radioactive decay and the positions of electrons operate according to probability - where different future outcomes are genuinely possible at a given time. This introduces "randomness"/chaos (i.e. non-determinism) into the system, even if it can be modeled according to a probability, and even if its mechanism/cause is fully identified and understood. "Randomness" does not necessarily mean "mystery".
One could imagine, for example, a mechanism which produces particle A 80% of the time, and particle B 20% of the time. Over a sufficiently long period of time, one expects the produced particles to be split roughly 80%/20% in favor of A. However, at any given time, there is uncertainty as to what it is going to do. And, if a simulation were run 10,000 times with this mechanism, the order of production would be likely be different in every instance
A decision is "determined" if the decider had zero agency in making it. A decision is (at least partially) "free" if the decider had agency in making it
In order to be said to have "agency" in making a decision, there would need to be some cause identified with the decider, which is not purely a product of prior deterministic causes. It would need to be something fundamentally rooted in agency (i.e., the capacity of a conscious subject to originate an action)
Suppose God exists, and created the universe. Why did God create? The Theist would respond, "Because it was God's 'will'". Why does God have such a will? There is no further cause - God's will is the terminus of the explanation. God's will, therefore, would be the ultimate cause of God's actions. Therefore, God's actions would be neither deterministic, nor random, but rooted in agency
In this view, God would need to create a mechanism for other conscious subjects to have agency