According to Calvinists, how could Adam and Eve fall? How could Satan fall? Under Calvinism, men under grace are guaranteed to choose God and be saved. 

If God's holy influence is so convincing, that it is impossible for free thinking creations to choose against it, why did something go wrong in the first place?
One prominent Reformed theologian, Louis Berkhof, has written that "[t]he problem of the origin of the evil that is in the world has always been considered as one of the profoundest problems of philosophy and theology." Other Reformed theologians share this assessment.  Charles Hodge suggests that given "the feebleness of our powers" and the "limited range of our vision" that it would be reasonable to "leave this question to be answered by God himself."2 Nonetheless, to answer what he sees as false doctrines, Hodge and others strive to provide an explanation.  

We should note that this question is closely related to the general problem of the existence of evil.  I will attempt to focus on the specific questions asked, regarding the fall of Adam and Eve and the demons.

Reformed theologians make several points: 1) that God is not the author of evil, 2) God's creation was good, 3) that Man and demons sinned voluntarily, and 4) God ordained that sin would come into the world.

God is not the author of evil

The first point emphasized is that God cannot be considered the author of evil.  Wayne Grudem writes that "we must clearly affirm that God himself did not sin, and God is not to be blamed for sin."3 Among the many passages cited in defense of this statement is Deuteronomy 32:4:


  The Rock, his work is perfect, for all his ways are justice. A God of faithfulness and without iniquity, just and upright is he. (ESV)


Berkhof writes that we cannot "make God the cause of sin in the sense of being its responsible author.  This idea is clearly excluded by Scripture."4  

Creation was originally good

Berkhof takes time to explain the original states of the angels and of men.  Citing Genesis 1:31, "God saw everything that He had made and, behold, it was very good," Berkhof argues that angels had an "original good condition," and notes that the good angels are called "elect" in 1 Timothy 5:21.  These, he says, must have received a "special grace of perseverance," which the fallen angels did not receive.5

Regarding man's original state, Berkhof calls it one of "relative perfection," like a child compared to an adult, "destined to reach a higher degree of perfection in the way of obedience." He was endowed with "original righteousness," but he "could lose it and still remain man."6 This original righteousness, according to Hodge, comes from man being created in God's image, and "includes not only his rational nature, but also knowledge, righteousness, and holiness."7

Covenant of Works vs. Covenant of Grace

One thing worth noting here is that prior to the Fall, Reformed theologians teach that Adam and Eve lived under a Covenant of Works, that is, a Covenant under which they were required to perfectly obey the law.  The punishment for failure was (obviously) death, but the reward for obedience was eternal life. Under this Covenant of Works, says Berkhof,


  [Adam] was not yet raised above the possibility of erring, sinning, and dying.  He was not yet in possession of the highest degree of holiness, nor did he enjoy life in all its fulness.  The image of God in man was still limited by the possibility of man's sinning against God, changing from good to evil, and becoming subject to the power of death.8


However, the reward for obedience in the Covenant of Works was eternal life.  Berkhof calls the time before the Fall a period of "probation, in order to determine whether he would willingly subject his will to the will of God."8  If Adam had obeyed, he and his descendants would have obtained eternal life.

God's relation to Adam under the Covenant of Works is different, therefore, from his relation to mankind under the post-Fall Covenant of Grace, which God made with sinners.  In the Covenant of Grace, say the Reformed, there are elect and non-elect, and the elect are predestined to receive grace ("irresistible grace"). Thus, Adam being "elect" under the Covenant of Grace would only make sense after the Fall.

Man and demons sinned voluntarily

Turning then to Adam's sin, Berkhof says that Adam's transgression was "a perfectly voluntary act on the part of man."9  Explaining God's command to not eat from the tree, Berkhof writes:


  [I]n order that the test of Adam might be a test of pure obedience, God deemed it necessary to add to the commandments of which Adam perceived the naturalness and reasonableness, a commandment which was in a certain sense arbitrary and indifferent.  Thus the demands of the law were, so to say, concentrated on a single point.  The great question that had to be settled was, whether man would obey God implicitly or follow the guidance of his own judgment.8


Other reformed theologians are also quick to state that man and fallen angels sinned voluntarily.  Hodge says that Adam's sinfulness was "self-acquired," and that the fallen angels "kept not their first estate."10 Grudem writes:


  It was man who sinned, and it was angels who sinned, and in both cases they did so by willful, voluntary choice.3


God ordained sin

In light of the previous points, it may seem surprising that Reformed authors continue by saying that God ordained sin to come into the world.  Grudem, citing Ephesians 1:11 and Daniel 4:35, writes that God


  did ordain that sin would come into the world, even though he does not delight in it and even though he ordained that it would come about through the voluntary choices of moral creatures.3


The natural follow-up question to this is, "why?," which gets into the general problem of evil.  I'll briefly mention, however, Hodge's answer to this question.  He argues that the glory of God is the ultimate purpose of creation, and that "there could be no manifestation of his mercy without misery, or of his grace and justice, if there were no sin."11 Citing Romans 9:22-23, where Paul discusses God's creation of vessels of wrath and vessels of mercy, Hodge concludes:


  Sin, therefore, according the Scriptures, is permitted, that the justice of God may be known in its punishment, and his grace in its forgiveness.  And the universe, without the knowledge of these attributes, would be like the earth without the light of the sun.11


Conclusion

Ultimately, Reformed theologians admit the difficulty of the problem, but hold these points as the clear teaching of Scripture and thus accept them.  Hodge recommends that we "rest satisfied with the simple statements of the Bible" and "with the assurance that the Judge of all the earth must do right."2 Similarly, John Calvin closes his treatment of the problem of evil by saying that "[o]ur true wisdom is to embrace with meek docility, and without reservation, whatever the Holy Scriptures, have delivered."12



References:


Berkhof, Systematic Theology
Calvin, Institutes of Christian Religion
Grudem, Systematic Theology
Hodge, Systematic Theology


Berkhof, 2.2.1, page 219
Hodge, 1.5.13, page 430
Grudem, page 492
Berkhof, 2.2.1, page 220
Berkhof, 1.2.4, page 145
Berkhof, 2.1.3, page 209
Hodge, 2.5.3, page 102
Berkhof, 2.1.4.C, pages 215-17
Berkhof, 2.2.1, page 221
Hodge, 2.5.6, page 113
Hodge, 1.5.13, page 435
Calvin, 1.18.4