Original Sin was Infidelity
Speaking on the fall on man, Calvin in his Institutes of the Christian Religion places infidelity and faithlessness at the center of man's rebellion:
In fine, infidelity opened the door to ambition, and ambition was the parent of rebellion, man casting off the fear of God, and giving free vent to his lust. Hence, Bernard truly says, that, in the present day, a door of salvation is opened to us when we receive the gospel with our ears, just as by the same entrance, when thrown open to Satan, death was admitted. Never would Adam have dared to show any repugnance to the command of God if he had not been incredulous as to his word. The strongest curb to keep all his affections under due restraint, would have been the belief that nothing was better than to cultivate righteousness by obeying the commands of God, and that the highest possible felicity was to be loved by him. (Inst. II.I.III) [Empahsis Added]
At its core, infidelity to God's word is the source of the fall. As Calvin summarizes, "when the word of God is despised, all reverence for Him is gone." Yes, the original failure from Adam manifested itself in ambition and rebellion but was rooted in faithlessness. Adam in suspecting God's word revealed his lack of faith. Hence Calvin can say:
The prohibition to touch the tree of the knowledge of good and evil was a trial of obedience, that Adam, by observing it, might prove his willing submission to the command of God. For the very term shows the end of the precept to have been to keep him contented with his lot, and not allow him arrogantly to aspire beyond it. The promise, which gave him hope of eternal life as long as he should eat of the tree of life, and, on the other hand, the fearful denunciation of death the moment he should taste of the tree of the knowledge of good and evil, were meant to prove and exercise his faith. (Inst. 2.1.3) [Emphasis Added]
Reliance and trust upon God are at the root of all God's dealings with man. Faith is the primary condition worked out in elementary obedience.
