The dominant theme in C. S. Lewis's The Screwtape Letters is the perpetual battle between God and Satan. His epistolary novel portrays human life as caught up in this battle, and either God will win a particular human life or Satan will. Though the letters of Screwtape, the senior devil, to his nephew Wormwood, a young devil in training, are intended to offer advice on how to steer a human soul away from God and towards Satan, they also show how a human soul's relationship with God becomes solidified. Lewis uses the letters to show how man's relationship with God can become solidified by incorporating Christian apologetics into each letter. Apologetics is the use of information and logic to defend points of faith.
Letter 27 deals with the common Christian difficulty of being distracted from God. Wormwood has written to Screwtape that his patient has fallen in love, which is a good thing for their side since love will distract the patient from God. However, Screwtape explains that distraction in and of itself is not a bad thing so long as the patient recognizes that it is a problem. Any sin that can be brought before God will actually draw a person closer to God, not farther away, or as Screwtape phrases it, "Anything, even a sin, which has the total effect of moving him close up to the Enemy, makes against us in the long run." Hence, Screwtape is pointing out to Wormwood that the fact that the patient is recognizing being distracted from God by his feelings of love and praying about them is a dangerous thing; distractions are only beneficial for Satan's side if they are ignored.
By distraction, we mean that when a person is in love, that person is thinking only of physical, worldly pleasures, not about the treasures one would gain in Heaven by following God's tenants. As Screwtape points out in the next paragraph, the fact that the patient has fallen in love is hopeful for Satan's side because it means that, since the patient is only focusing on worldly pleasures, most of his prayers to God are now petitionary, which means he is mostly requesting things of God when he prays. He is most likely requesting that his beloved will be protected, that she will love him in return, and that they'll be joined in marriage. Screwtape continues to argue that petitionary prayers can be used to turn souls away from God because souls can be led to believe that petitionary prayers are useless and undermine free will.
Screwtape connects the uselessness of petitionary prayers to God's all-seeing ability. All throughout the book, Screwtape emphasizes the need to get the patient to think illogically since sound reasoning will lead a person to God. One fallacy Screwtape advocates Wormwood use in this letter is the idea that because God is all-seeing, he already sees the future. Since he already sees the future, he already knows what people will ask for in their prayers. Hence, if a prayer request is granted, it wasn't necessarily granted as a prayer; it was granted simply because it was going to happen anyway. The second fallacy God's all-seeing nature undermines free will: Souls aren't praying their requests before God out of their own free will; they are doing so because they were "predestined to do so."
But, again, all of the above arguments are logical fallacies, and if Wormwood can get his patient to think along these lines, Wormwood will lead the patient towards Satan. As Screwtape further explains, being all-seeing does not mean that God sees the past, present, and future in the same way man sees them. Instead, as he points out that Boethius, a 6th century Roman philosopher, has already figured out, being all-seeing means that God sees everything as happening "in his unbounded Now." Plus, seeing things as happening now is not at all the same as forcing someone to do something now. Therefore, petitionary prayer is neither useless nor an undermining of free will.
However, as Screwtape informs Wormwood, if Wormwood can make the patient believe his petitionary prayers for his loved one are both useless and an undermining of his free will, then Wormwood can capture his soul for Satan.
Each letter in The Screwtape Letters can be interpreted in exactly the same way. First, we identify the potential sin or false way of thinking that Screwtape is speaking of; then, we identify the apologetical arguments Lewis is making through Screwtape to explain the logical fallacy and to direct a reader's reasoning towards God.