Faith is the art of holding on to things your reason has once accepted, in spite of your changing moods. . . . That is why faith is such a necessary virtue. Unless you teach your moods "where they get off," you can never be either a sound Christian or even a sound atheist, but just a creature dithering to and fro, with its beliefs really depending on the weather and the state of its digestion.
The first step is to recognize the fact that your moods change. The next is to make sure that, if you have once accepted Christianity, then some of its main doctrines shall be deliberately held before your mind for some time every day. That is why daily prayers, and religious reading, and church going are necessary parts of the Christian life. We have to be continually reminded of what we believe. Neither this belief nor any other will automatically remain alive in the mind. It must be fed. And as a matter of fact, if you examined a hundred people who had lost their faith in Christianity, I wonder how many of them would turn out to have been reasoned out of it by honest argument? Do not most people simply drift away?
(p. 123, 124 Mere Christianity)