Theological determinism of the sort affirmed in classical Calvinist theology holds the following principle: the will and voluntary determination of God is causally and logically prior to the contingent historical occurrences which take place thanks to God's providence. Thus if a certain person turns out sinful, dies unreconciled to God and inherits eternal damnation, God's own will that this should be so is both logically and causally prior to this event itself: God's creation of the world and its preservation in existence is clearly the causally prior condition; but also God's will is logically prior, in that God did not merely foreknow that this would happen but instead decided that it would, without acting on the basis of foreknowledge of causally independent creatures' choices.
This view is arguably entailed by the classical theistic conception of God. On this view, God is omniscient, and this means he knows what people are going to do in the future. He knows what I will do tomorrow, and whether I will be saved or lost, etc. But God is also characterized by aseity, which means that he owes the perfection of his being to nothing outside of him. God isn't strong by virtue of something outside of him, for example, but simply is omnipotent in himself. But now if God is omniscient and knows what I will do in the future, and if by his aseity he doesn't know this through anything outside of himself, then presumably the only way he can know it is by knowing himself—or more specifically, by knowing what he has willed to happen with me. So classical theism seems to entail theological determinism.
This wouldn't be so bad except for the fact that God evidently will judge us for the good and evil that we do. If you hold, for example, that Jones is going to die and be punished in hell for his sins, the theological determinist has also to maintain that God's will is both causally and logically prior to this event. His will ultimately explains why it will happen. The problem now is this: it is contrary to the phenomenology of moral accountability and responsibility that one person should punish another for the latter's actions, so long as the former's will stands in an explanatory relation of total causal and logical priority to the latter's action. If Smith's volition is both causally and logically prior to Jones's crime (say he hypnotized Jones and convinced him to want to kill his wife as well as to go through with it), then it is impossible for Smith rightly and justly to punish Jones for this particular crime. In the same way, on the theological determinist picture, God's will is causally and logically prior to every facet of a human person's existence and history; the whole human person is only exactly that which God wanted her to be. For this reason, it can never be just for God to punish her for things she has done wrong.
The key issue here is that of logical priority. If a know a person is liable to commit a crime under certain circumstances, I may work to bring about those circumstances so as to catch them in the act. And I could rightly punish them for what I catch them doing. But this presupposes that my own volition is not logically prior to theirs, since the success of my scheme depends intrinsically on the free cooperation of the other person. There is an essential element of spontaneity and a lack of total control that is absolutely essential to genuine moral relations. As soon as I assume a position of logical priority to the volition of the other person—if I hypnotize them or take control of their brain through an implant or whatever—then my ability to punish is fatally compromised. Likewise, if another person should have stood in such relations of priority to my own volition as regards some particular action, then I rightly renounce all responsibility for it—regardless of whether I wanted to do it, or whether I satisfy whatever other conditions of compatibilist notions of freedom.
This is why Calvinism and other deterministic theological paradigms are to be rejected. So long as we hold that God's will is logically and causally prior to the will of the creature in every way, there is no longer any justice in punishing sinners, nor is there any genuine fellowship between God and humankind more generally.
Put another way, this is the central and most important "problem of evil" for deterministic theology: the phenomenology of moral relations makes it impossible for God justly and rightly to punish sinners, so long as his will is logically and causally prior to the volition of the agent. On the contrary, there now seems to be no conceivable, justifiable reason for God to do such a thing in the first place. It seems to speak more than anything of pure and unmotivated malice.
Suppose someone says that God decrees the damnation of the reprobate for the sake of the elect, so that they might know God's mercy better as well as his justice. On this view, God still exhibits an apparently arbitrary malice towards the reprobate considered in themselves, who are ultimately as they are simply because God decided they would be. Even if they should, perhaps through some sort of perversity of their own will, agree that they deserve eternal punishment or even desire it, this too would still be ultimately explained by God's logically and causally prior will. The potential for that kind of pure malevolence is despicable in itself, something no one would conceive of as possible for even a good person, let alone God, and so cannot rightly be attributed to God without blasphemy.
For this reason, too, deterministic theology is to be rejected. If the alternative is maintaining an apparently irreconcilable tension between classical theism and genuine human freedom, that is preferable. Perhaps there are simply some mysteries that cannot be known. Better that than to describe God in these terms.