The first part of Milton's argument hinges on the word disobedience and its opposite, obedience. The universe that Milton imagined with Heaven at the top, Hell at the bottom, and Earth in between is a hierarchical place. God literally sits on a throne at the top of Heaven. Angels are arranged in groups according to their proximity to God. On Earth, Adam is superior to Eve; humans rule over animals. Even in Hell, Satan sits on a throne, higher than the other demons.
This hierarchical arrangement by Milton is not simply happenstance. The worldview of the Middle Ages, Renaissance, and Restoration was that all of creation was arranged in various hierarchies. The proper way of the world was for inferiors to obey superiors because superiors were, well, superior. A king was king not because he was chosen but because he was superior to his subjects. It was, therefore, not just proper to obey the king; it was morally required. Conversely, if the king proved unfit or not superior to his subjects, it was morally improper to obey him and revolution could be justified.
God, being God, was by definition superior to every other thing in the universe and should always be obeyed. In Paradise Lost, God places one prohibition on Adam and Eve — not to eat from the Tree of Knowledge. The prohibition is not so much a matter of the fruit of the tree as it is obeying God's ordinance. The proper running of the universe requires the obedience of inferiors to their superiors. By not obeying God's rule, Adam and Eve bring calamity into their lives and the lives of all mankind.
