I also; at which time this powerful Key
Into my hand was giv’n, with charge to keep [ 755 ]
These Gates for ever shut, which none can pass
Without my op’ning.
The key to the gates of Hell falling into the hand of Sin and Milton’s emphasis on the presence of choice in Hell proposes an idea surrounding the theology Milton is seeking to explain or detail in Paradise Lost. By placing the key in the hand of sin, this allegorical figure, he is stating that a person would have to have sin in order to enter Hell. Each creature has been created good by an all good God, but he leaves them with this freedom to chose his goodness or not. This is because of the idea that without the choice one is simply an object not a soul. When a creature chooses to act in a way contrary to God, that is sin. Milton is using the symbol of the key as a sign of agency as well it seems, one as the choice to remain with God or disobey God or sin. This sin is what opens the gate. Milton very carefully ties in these symbols of agency, I think, to answer the common questions about Christianity like why would a good God create bad things and why would a good God send his people to suffer in Hell?. It appears Milton is saying that at one point, at least, each soul had a choice between the staying near God and grabbing the keys to Hell.