Man has quite a struggle through life; every day, he is a player in the struggle between Light and Dark, Good and Evil, God and Devil. This is not just any struggle, either. This is the struggle for the soul of each individual man and, therefore, is the absolute toughest challenge that man will face in life. However, we do have one advantage... we (hopefully) choose to turn ourselves towards Good and, therefore, have all the strength of Good on our side. The common theme in the victory over adversity is one's reliance on God, or gods
In The Screwtape Letters, two devils (Screwtape, undersecretary to Our Father Below and Wormwood, his nephew) strike up a correspondence and try to work out a way to tempt a young man to Hell. However, the young man turns to a reliance on God in order to give him the grace to persevere through the adversity his is put through and triumphs, ultimately, due to this reliance.