From his book Mere Christianity, I quote: When a man who accepts the Christian doctrine lives unworthily of it, it is much clearer to say he is a bad Christian than to say he is not a Christian.
Now, it would appear that Lewis did not like to call the bluff on the profession of faith, but then he also said in the same book:
If conversion to Christianity makes no improvement in a man's outward actions – if he continues to be just a snobbish or spiteful or envious or ambitious as he was before – then I think we must suspect that his 'conversion' was largely imaginary.