Here we have a fight on our hands and God is watching from the stands. Our fight is with all our vices and Satan, the prince of vices. He challenges our soul to a single combat. Whatever evil or unlawful thing may be suggested to your thoughts, whatever dark, unwholesome desire wells up from your flesh against your mind, these are the weapons of the enemy who is challenging you to combat. Your foe and your protector are both invisible. You cannot see the Satan with whom you are locked in combat but you can know through faith the God who protects you.

—Augustine, Sermon 335K.3