Letter 16 and 17 are masterful ways in which Screwtape advises Wormwood to treat the "patient." Just in case you did not know, these letters are from a higher level demon to a lower level demon to hinder people from deepening their faith. 

In letter 16, Screwtape advises Wormwood to get his subject to take the stance of a consumer. If a person church hops or shops, then he or she will become a critic rather than a worshipper. This is a great point, because a critic will not grow in faith. Moreover, Screwtape urges Wormwood to send him either to a liberal church whose minister does not believe in the Bible or a radically conservative church that believes in the Bible with literal precision. Both are distortions of orthodox Christianity, and both do not understand the importance of unity. Here is an excerpt:

Surely you know that if a man can't be cured of churchgoing, the next best thing is to send him all over the neighbourhood looking for the church that "suits" him until he becomes a taster or connoisseur of churches.

In letter 17, Screwtape teaches Wormwood the power of gluttony. Gluttony does not have to be a desire for excessive amounts of food or things but a commitment to a person's appetite. If a person's appetites are the highest priority, then the person is a slave to himself and will never become a maturing Christian. Keep in mind a strong strand in Christian theology is about denying appetites. 

Here are the words of the letter:

The woman is in what may be called the "All-I-want"  state of mind. All she wants is a cup of tea properly made, or an egg properly boiled, or a slice of bread properly toasted. But she never finds any servant or any friend who can do these simple things "properly"—because her "properly" conceals an insatiable demand for the exact, and almost impossible, palatal pleasures which she imagines she remembers from the past...