Because what God so severely punished must have been no light sin but a detestable crime, we must consider what kind of sin there was in Adam’s desertion that enkindled God’s fearful vengeance against the whole of mankind. To regard Adam’s sin as gluttonous intemperance (a common notion) is childish. As if the sum and head of all virtues lay in abstaining solely from one fruit, when all sorts of desirable delights abounded everywhere; and not only abundance but also magnificent variety was at hand in that blessed fruitfulness of earth! We must, therefore, look deeper than sensual intemperance. The prohibition to touch the Tree of Knowledge of good and evil was a trial of obedience (obedientiae examen), that Adam, by observing it, might prove his willing submission to the command of God.
…The promise, which gave him hope of eternal life as long as he should eat of the tree of life (arbore vitae), and, on the other hand, the fearful denunciation of death the moment he should taste of the Tree of Knowledge of of good and evil, were meant to test and exercise his faith.
—John Calvin (1509-64), Institutes of the Christian Religon (1559), 2.1.4.