I also found what he said about the patient being a churchgoer being a good thing for the devil's cause to be very true. It's a common misconception that those who go to church every Sunday are more "Christian" than other Christ-followers. Yes, going to church can build and strengthen one's relationship with God in amazing ways, however, some make it their only form of weekly worship. Many Christians just go out of habit or because they believe it is expected of them, or, as Screwtape explains it, go because in retaining the external habits of a Christian one can believe that they truly are so.
Basically, Screwtape is saying that if we remain stagnant and apathetic about our faith, we are allowing ourselves to become vulnerable to whatever the devil throws our way. If we are not living out our faith every day we will not be completely fixed upon the Lord and will then be subject to whatever the devil may use. We are all sinners by nature, so anyone who doesn't make faith their number one priority is likely to falter. It's a common misconception that we must commit an unbelievable sin to be condemned to hell when all it takes is an inactive faith. "Indeed, the safest road to Hell is the gradual one--the gentle slope, soft underfoot, without sudden turnings, without milestones, without signposts."