Theodicy
Definition
From Greek: theos means God, dike means justice. Theodicy is that branch of theology which explains evil by justifying God for it's existence (evil's).
Different Approaches
One of the earliest approaches on theodicy is that of St. Irenaeus (c. 120-203).The Irenaean tradition perceives the Fall, Adam and Eve's original sin, from an evolutionary point of view. Evil and good are fused together and man can learn by mistakes and sins. This would lead him closer to perfection.
St. Augustinus (354-430) offers another explanation to the Fall. In his model, evil is seen either as a punishment for the sin or as a consequence of the disturbance of the natural world, which was first established by God.
Thomas Aquinas, a 13th-century theologian, argued that evil as such does not exist; under "evil" is to be understood the lack of good.
A modern and very important approach for theology and philosophy in general, is that of the German Philosopher, Gottfried Wilhelm Leibniz, which is treated in his book Theodicy (1710). He categorises evil into three forms: moral, physical and metaphysical. According to Leibniz, our world is the best model of all the worlds that God could have created; as God wants the best for man, he gave us the best possible world and the closest one to perfection.
In almost all the approaches on theodicy, free will of man plays the crucial role for the existence of evil and sin.
Milton and Theodicy
John Milton dealt with the topic of theodicy in his two famous epic poems: Paradise Lost and Paradise Regained. Besides dealing with the biblical question itself, his works, especially the former, might have reflected the situation of puritans in England after the Restoration of Charles II.
References
Milton's Religious Context. 02. Jul. 2009. http://www.christs.cam.ac.uk/darknessvisible/religion.html
Theodicy. 02.Jul. 2009. http://www.britannica.com/EBchecked/topic/590596/theodicy?view