When Christians speak about faith, they usually mean one of two things. First, “faith” refers to belief in the truth of the doctrine of Christianity. It’s somewhat surprising that faith in this sense could be considered a virtue—how could there be any virtue in one’s ability to believe? But Lewis argues that belief also takes a tremendous amount of willpower. For example, when Lewis is about to undergo a painful surgery, he has to force himself to continue believing that the anesthetics will keep him from feeling pain. By the same token, it takes effort to keep up one’s faith in Christian doctrine.
Faith could be considered a precondition for Christian virtue, rather than a virtue itself. In other words, people have to believe that their good behavior has some greater meaning, some consequence—acceptance in Heaven—before they embark on a lifetime of morality. The problem here is that even the most devout Christian wavers in their belief sooner or later—it is human to doubt (particularly since God doesn’t show himself to human beings in concrete ways, as Lewis discussed in Book One).
Even if readers believed everything Lewis has said in his book so far, they might not necessarily become a Christian—the following week, they could get some bad news and decide to abandon their new faith altogether. Faith in Christianity takes conscious effort—that’s why there are lots of “convenient Christians” who are only religious when things are going well. Put differently, faith is the ability to hold “onto things your reason has once accepted, in spite of your changing moods.”
As we near the final portion of Mere Christianity, Lewis goes over some caveats in his argument thus far. It is not enough to just accept the logic of Lewis’s writing; one must also make the leap of faith and choose to live by that faith. There is, in short, a huge difference between accepting Christianity (i.e., recognizing that there is a God and that Jesus Christ died for mankind’s sins) and living Christianity.
How does one hold onto faith over time? The first step is to accept that one’s mood changes constantly. The next is to engage in Christian practice. Prayer, worship, and religious readings are vital parts of Christianity because they remind people that their beliefs remain strong.
One of the reasons that prayer and ritual are vital parts of Christianity is that humans need concrete, real-world reminders of the strength of their religion—and rituals provide such a reminder.
To begin to understand the second sense of the word “faith,” consider someone who’s newly converted to Christianity. It can be very difficult to keep the faith for a long time. Even very good Christians live in constant danger of being tempted into sin. Thus, a new Christian convert must accept how hard it is to be good. Furthermore, new Christians must realize that everything they do is “given” to them by God. It is impossible to give God anything that was not his to begin with. In a sense, then, a human practicing Christian virtue is like a child asking his father for some money to “buy” the father a birthday present. Once a new Christian accepts these basic truths about virtue, “God can really get to work.” Lewis will then explore the second definition of faith in the following chapter.
Ultimately, it is not enough to just live a good, moral life—even the most devout Christian will sin sooner or later. Thus, the strict truth is that human beings don’t “deserve” to go to Heaven—even a very, very moral human being is a sinner. Furthermore, “good Christians” must accept that their accomplishments and achievements in life don’t truly “belong” to them—God provided them. Good Christians must accept, in short, that their virtue is really God’s, and their sins are theirs alone.