In discussing Adam we are at the disadvantage of the Biblical evidence is at a minimum. We agree that Libertarianism fails to answer the question as well. That nobody is free from the eternal decrees of Lord Jesus. We have alternative explanations for why Adam sinned. The origin of Sin is a difficult issue, but possible. We have the Edwards-Anderson(mysterian) response and another more uncommon one. Edwards in the Freedom of the Will wrote:
Concerning sins first Entrance into the world.
The things which have already been offered, may serve to obviate or clear many of the objections which might be raised concerning sin’s first coming into the world; as though it would follow from the doctrine maintained, that God must be the author of the first sin, through his so disposing things, that it should necessarily follow from his permission, that the sinful act should be committed, &c. I need not, therefore, stand to repeat what has been said already, about such a necessity not proving God to be the author of sin, in any ill sense, or in any such sense as to infringe any liberty of man, concerned in his moral agency, or capacity of blame, guilt, and punishment.
But, should it nevertheless be said, that if God, when he had made roan, might so order his circumstances, that from these, together with his withholding further assistance and divine influence, his sin would infallibly follow, why might not God as well have first made man with a fixed prevailing principle of sin in his heart?
I answer, 1. It was meet, if sin did come into existence, and appear in the world, it should arise from the imperfection which properly belongs to a creature, as such, and should appear so to do, that it might appear not to be from God as the efficient or fountain. But this could not have been, if man had been made at first with sin in his heart; nor unless the abiding principle and habit of sin were first introduced by an evil act of the creature. If sin had not arisen from the imperfection of the creature, it would not have been so visible, that it did not arise from God, as the positive cause, and real source of it.—But it would require room that cannot be here allowed, fully to consider all the difficulties which have been started, concerning the first Entrance of sin into the world.—And therefore,
2. I would observe, that objections against the doctrine that has been laid down, in opposition to the Arminian notion of liberty, from these difficulties, are altogether impertinent; because no additional difficulty is incurred, by adhering to a scheme in this manner differing from theirs, and none would be removed or avoided, by agreeing with, and maintaining theirs. Nothing that the Arminians say, about the contingency or self-determining power of man’s Will, can serve to explain, with less difficulty, how the first sinful volition of mankind could take place, and man he justly charged with the blame of it. To say, the Will was self-determined, or determined by free choice, in that sinful volition—which is to say, that the first sinful volition was determined by a foregoing sinful volition—is no solution of the difficulty. It is an odd way of solving difficulties, to advance greater, in order to it. To say, two and two make nine, or, that a child begat his rather, solves no difficulty: no more does it, to say, the first sinful act of choice was before the first sinful act of choice, and chose and determined it, and brought it to pass. Nor is it any better solution, to say, the first sinful volition chose, determined, and produced itself; which is to say, it was before it was. Nor will it go any further towards helping us over the difficulty, to say, the first sinful volition arose accidentally, without any cause at all; any more than it will solve that difficult question, How the world could be made out of nothing? to say, it came into being out of nothing, without any cause; as has been already observed. And if we should allow, that the first evil volition should arise by perfect accident, without any cause; it would relieve no difficulty, about God laving the blame of it to man. For how was man to blame for perfect accident, which had no cause, and which, therefore, he was not the cause of, any more than if it came by some external cause?—Such kind of solutions are no better, than if some person, going about to solve some of the strange mathematical paradoxes, about infinitely great and small quantities—as, that some infinitely great quantities are infinitely greater than some other infinitely great quantities; and also that some infinitely small quantities, are infinitely less than others, which yet are infinitely little—should say, that mankind have been under a mistake, in supposing a greater quantity to exceed a smaller; and that a hundred, multiplied by ten, makes but a single unit.
Dr. James Anderson in the book “Calvinism and the Problem of Evil” maintained the traditional Edwards response.
The other response is from Ronald W. Di Giacomo and I recommend you read his whole article and then his whole blog.
God is not a legalist:
If Adam intended to act sinfully and was tackled prior to acting upon his intention, wouldn’t he have sinned just the same? Moreover, had Eve abstained from eating the forbidden fruit solely because she was concerned for her figure, would she not have sinned just the same in the eyes of God? Certainly God is not a legalist who overlooks the intentions of the heart! Consequently, the sin of eating came from a sinful intention that had occurred prior to the visible act that followed from that intention.
Mystery, mystery when there is no mystery:
The reason people call the first sin a mystery is because they begin their reasoning with the false premise that the act of taking and eating the forbidden fruit was the first sin. If we get back to first principles and focus on what precedes any volitional act, whether sinful or not, we can begin to recognize that the first sin was the desire to be like God and not the act that proceeded from that desire. Accordingly, the first sin was Adam’s nature upon becoming fallen, which correlates with his desire to be like God. Adam, in other words, had concupiscence prior to acting sinfully. To deny that Adam’s first sinful act came from a nature that had already fallen is to affirm that a sinful act came from a non-sinful nature, a monstrosity indeed. The question that we should be concerned with is not how did an unrighteous act spring from an upright being (which is a question that proceeds from a false premise), but rather how did an upright being acquire a sinful intention to act sinfully? The answer is no different than the answer to the question of how does any intention and subsequent act come into existence. Doesn’t God providentially orchestrate circumstances that come before the souls of men thereby moving them by secondary causes to act in accordance with new inclinations that are brought into existence according to God’s providence that He decrees? By God’s pre-interpretation of the otherwise brute particulars of providence, the intentions of men and their subsequent acts fall out as God so determines. For Calvinists to argue that an act of sin proceeded from an upright nature is to assert a contradiction – and no amount of mystery can save a contradiction! The only thing I find mysterious is that so many Calvinists find the entrance of sin into humanity so mysterious. Note well that I am not pretending to know how God pre-interprets particulars or how the mind of man relates to the movement of the body. That’s not in view at all. My simple point is that Calvinists do not generally find it mysterious that volitional acts necessarily follow from intentions and that God’s orchestrating of circumstances are an ordained means by which intentions that never existed before come into being. Why, therefore, should we not apply the same theological reasoning to the first sin as we do to God’s sovereignty over the intentions of fallen men? The mystery is the same. We don’t know the details of how God brings to pass the intentions of the heart, but that is not peculiar to the first sin. It pertains to all intentions. Again, had Adam been tackled prior to eating the fruit, wouldn’t his intention to eat have been sin? And wouldn’t that intention have come from a fallen nature? Now did his intention to eat somehow not become sin because he was not tackled and actually did act according to his intention? Of course not! His sin was the intention of his heart (which could have only come from a nature that could produce such an intention), and he also sinned by acting on that intention. So the first sin was the fallen nature and the desire to be like God, then the intention to act and then the subsequent actions. Now is any good Calvinist going to say that we choose our intentions or our nature? No, but we are certainly responsible for them, for they are ours!