Lets me begin with two insights:  C.S. Lewis was not a Baptist and did not believe in eternal security. The devil can destroy your fellowship but not your relationship. That would be giving him far too much credit.  The devil doesn’t get in a hurry. It’s like the story of the little lady that never said anything bad about anyone. The pastor decided that he would trick her so he asked her in front of a group, “Madam, what to you have good to say about the devil?” She paused and then said, “Well, I tell you this, he stays busy.” She was right, he stays busy and he is patient. In Letter 12, Screwtape is urging Woodworm to slow his pace. Screwtapes advice is…”Don’t hurry the patient, there is always the chance that you will awake him.”
The devils scheme is to slowly move the believer away from the fellowship with God or in the case of a non-believer, slowly away from a point of repentance. The patient must not be allowed to suspect what is actually going on. The devil does not want him/her to understand the digression that is taking place. Slowly, in increments, the devil is leading the patient further and further away or in most cases down. The devil does not want us to think clearly or to acknowledge the uneasiness in our soul. If people realize that they are unhappy there is always the danger that they will seek God or possibly repent. In church, the devil would have us be “At ease in Zion”. Outside of church, he would have us be content with no church at all. So long as we are content and see no need for change, he is well pleased.
So Satan would have us look away from our discontent and think little of our Creator and Christ. Satan gets nervous when we set our affections on things above. So he patiently urges us to move away from the Lord and he distracts us from thinking about the Lord. Ever so gradually, he leads us into temptation knowing that once we yield, the former temptations is transformed into a habit and habits are not near so much pleasure as temptation. Of course he knew all along that there was no real pleasure in the sin but he does enjoy seeing us stew in our own lustful juices. The lower we sink into sin and rebellion, the less pleasure we experience until we real a state of misery. Satan would be well pleased if all humans were miserable. Screwtape quotes one of hell’s latest arrivals as saying with regret, “I now see that I spent my life doing neither what I should have or what I really liked.”
Satan is a thief and he will steal our best years if we are not careful. He will do it by increments, urging us step by step into a gradual decline that will lead to a state of apathy. At this point we get lost in a maze of daydreams. We have no real sense of reality and very little ambition to change.
One final point: We have to remember that Satan uses hate just as he does murder. He uses lust as if it were adultery; in some ways it serves his purpose better. Sometimes one shocking sin will wake a man up. The sin is not the issue with him, that is a social stigma that we have, Satan has no such qualm. If he can get you addicted to reading the newspaper or watching the TV rather than reading your bible, he will accomplish his goal. Most humans are intelligent enough to avoid the most wicked crimes. The thought of murder horrifies us but a deck of cards, what could be the harm? Satan uses the things to which we give little thought and of which we have little fear. That is what makes him a devil.