i) When we discuss theological traditions, the tendency is to concentrate on what's distinctive about that tradition. There may be individual distinctives, or there may be a distinctive package. We tend to focus on what differentiates that tradition from the alternatives. That can be misleading inasmuch as there's more, much more, to a theological tradition than what distinguishes one tradition from another.
ii) That said, let's consider the distinctive features. What is Calvinism? At the most general level, Calvinism takes the view that everything happens for a reason. Every event, whether physical or mental events, serve a purpose. Indeed, everything happens for a good reason, including–or especially–bad things. Some events may be intrinsically evil but instrumentally good.
iii) But what is necessary for everything to happen for a reason? In order for everything to be purposeful, to have an explanation, there must be a master plan, in which every event is coordinated in a part/whole, means-ends relation. Everything happens according to plan. God wrote the plot.
I should add that this isn't unique to Calvinism, but holds true for other predestinarian traditions (e.g. Thomism, Augustinianism, Jansenism).
If there are unpremeditated events, then everything doesn't happen for a reason. Some events are brute facts–like sheer luck, which can be good luck or bad luck. Pointless things happen. Tragedies happen that serve no purpose. By chance, the victim was in the wrong place at the wrong time.
When Calvinism says everything is predestined, that means everything happens for a reason. The alternative is that some, many, or most events have no specific rationale. In that regard, they are random events. Inexplicable events.
iv) In fairness, a freewill theist might say everything happens for a general reason: namely, the overarching value of libertarian freedom. But freewill theists typically denounce the idea that some tragedy or atrocity was "God's will". So they deny that every event–especially evil events–happens for a specific reason, or serves a particular purpose.
v) That's a definition of Calvinism at the most general level. Of course, that cashes out in more detailed terms. There's the particularism of grace. Unconditional election and reprobation. Limited atonement.
In theory, critics might not find Calvinism so objectionable if it merely took the view that everything happens for a reason, but in a world where evil occurs, they find that more principle more contentious. And they think reprobation is evil in its own right.
vi) One objection is that it's cruel for the Calvinist God to save only some people when he could save everyone. But bracketing other issues, that's equivocal. Let's pick a figure out of thin air for discussion purposes. Suppose, in the actual world, the elect are 70% of humanity while the reprobate are 30% of humanity. Could the Calvinist God save the 30% in addition to the 70% if he so chose?
That's far from clear. Although there are possible worlds in which everyone is elect, those have different genealogies than a world in which 70% are elect and 30% are reprobate. If the 30% were elect, they'd make different choices in life. They'd produce different family trees. It wouldn't be saving the same 30% in addition to the same 70%, for almost no one would be the same. In a world where everyone is elect, different people are born into that world due to the choices of their elect forebears.
The upshot is that none of the heavenbound people in a world where 70% are elect would even exist in a world where 100% are elect–assuming death seals your eternal fate. A critic might say the Calvinist God could still save the lost after death, but that moves the hypotheticals outside the boundaries of biblical orthodoxy.
vii) Another objection is that it's a miscarriage of justice for God to punish agents for sins he predestined them to commit. And that might strike many people as prima facie counterintuitive. However, it's often the case that we can't properly assess a potion in isolation. Rather, we need to compare to the alternatives.
What does it mean for human choices not to be predestined? When freewill theists say humans have libertarian freedom, does that mean our choices are ultimately uncaused?
Consider dice. Predestination is like loaded dice. The outcome is certain every time, ahead of time.
The alternative is fair dice. It's not that the outcome is strictly uncaused. The laws of physics apply.
Rather, each throw is causally independent of the preceding or succeeding throw. In that sense, the outcome is random or uncaused. Every time you throw the dice, it's like the first time. A particular outcome doesn't make the next outcome more or less likely. Each time you throw the dice, you might roll different numbers or the same numbers. So it's arbitrary in that regard. In effect, every throw is a fresh start, no matter how often you threw the dice.
This also means that inevitably, the results of throwing fair dice will sometimes coincide with the results of throwing loaded dice. Likewise, odds are that random choices will sometimes coincide with predestined choices. In that case, would it be unjust for God to punish an agent for a predestined choice of that coincided with a random choice?
viii) Conversely, is it just for God to punish an agent for a random choice? Suppose a psychopathic kidnapper took a man's wife and kids hostage. But he gives the man a chance to save his family by throwing dice. If the outcome is six or above, the kidnapper won't shoot them. If the outcome is below six, the kidnapper will shoot them.
But isn't that grossly unfair? The results of one throw are arbitrary inasmuch as each throw might be different. Why should the first and only throw be decisive?
If freewill theism is true, aren't our choices like that? If I roll the dice at noon, I'd get one outcome. If I roll the dice at 11:59, I might well get a different outcome. Likewise, if I roll the dice at 12:01. Yet the God of freewill theism holds me to one particular throw, even though it's by chance that any particular outcome occurs. Picking one particular throw out of a hypothetical sequence, where if the pick was sooner or later, the results would chance.
Suppose a free agent (in the libertarian sense) made a different choice than the predestined choice. But his actual choice, if random, is arbitrary.Given the opportunity to role the same dice multiple times, the results might differ every time. So why privilege or absolutize the actual choice? Isn't that an artificial sample? Why make that the cutoff when, if he repeated the trial under the same circumstances, the results might turn out differently? Why select for that particular throw as if that's somehow definitive?
ix) However, a freewill theist might object that I've caricatured libertarian freedom. An astute freewill theist will concede that we don't approach decision-making as blank slates. Although our choices may not be predetermined, there are factors that predispose us to opt for one choice rather than another.
On that view, the alternatives aren't confined to fair dice and loaded dice, because libertarian choice is more like throwing biased dice. Unlike loaded dice, which make one outcome inevitable, or fair dice, which make every outcome equiprobable, biased dice make some outcomes more likely than others.
But I don't see how that refinement helps the freewill theist. In that event, is it just for God to punish the agent for his choice unless the agent wasn't equally free to choose one thing rather than another? I'm not saying I agree with that. I'm just considering the libertarian position on its own grounds.