Just as a reminder, these questions were posed to C.S. Lewis on April 18, 1944 by workers at the Electric and Musical Industries Ltd., in Hayes, Middlesex. This company, known today as EMI, is probably best known for its recording studios at Abbey Road, London which was where the Beatles recorded their music. It's quite interesting to see how Lewis handles their questions, most of the time keeping his answers quite short.
We don’t qualify for heaven by practice, but salvation is obtained at the Cross. We do nothing to obtain it, but follow Christ. We may have pain or tribulation, but nothing we do qualifies us for heaven, but Christ.
The controversy about faith and works is one that has gone on for a very long time, and it is a highly technical matter. I personally rely on the paradoxical text: ‘Work out your own salvation. . . for it is God that worketh in you’ (Philippians 2:12). It looks as if in one sense we do nothing, and in another case we do a damned lot. ‘Work out your own salvation with fear and trembling,’ but you must have it in you before you can work it out. But I have no wish to go further into it, as it would interest no one but the Christians present, would it?
"Answers to Questions on Christianity," God in the Dock (Eerdmans: 1970) 55.