Light

A favorite metaphor for God during the Middle Ages was light.  Many of these metaphors still make sense, but some, like St. Thomas Aquinas' idea of light "illuminating" the air, do not survive our current physics.  We now understand that light travels even in a vacuum, not least between our sun and our own planet.  But this concept of light tells us a different lesson about God.

It is often difficult to see how God and evil can exist in the world.  How can God, who is infinite good, allow evil to remain?  If good and evil are two opposing forces, like the two ends of a magnet, then God's goodness would indeed push away all the evil in the world.  But evil is not the opposite of good, it is the lack of good.  So where pure evil exists, there is a complete vacuum of good.

God's infinite goodness does not attempt to fill the voids on his own.  He does not need to, because he can transcend them.  He penetrates through the vacuum and shines His goodness upon us, and if we are willing to follow Him, we begin to fill the voids.  This is God's grace.  God transcends and uses evil in order to teach us, so that we may become saints.