Well, you don't believe all the doctrines of the Bible, either, so what's the problem?
C.S. Lewis was very popular and did a lot of great things. He is remembered in the United States for the books that he wrote and the ideas he gives to apologetics but no one quotes him as an exegetical scholar. He had a lot of success and is quoted by a lot of well known and well respected Apologists like Josh McDowell and Dr. Ravi Zacherias.
I'm not the only one questioning C.S. Lewis:
http://www.equip.org/free/JAL400.htm
Dr. Norman Geisler says a lot of great things about Clive Staples Lewis in the Baker Encyclopedia of Christian Apologetics. I think Lewis' problem starts with his view on Inspiration. Geisler says that Lewis is neither Orthodox or Neo-Orthodox but calls Lewis a Liberal-evangelical and Geisler says that he uses the term as a paradoxical one.
Geisler gives a list of the various views on inspiration:
Extreme Fundamentalism - Verbal dictation through secretaries
Orthodox - Verbal inspiration through prophets
Liberals - Human intuition through natural process
Liberal-Evangelical - Divine elevation of human literature
Neo-Orthodox - Human recording of revelational events
Neo-Evangelicals - Inspiration of only redemptive truths or purpose
"According to Lewis,'the voice of God [is heard] in the cursing Psalms through all the horrible distortions of the human medium." p.176, "A General Introduction to the bible" by Dr. Norman Geisler (C.S. Lewis, Reflections on the Psalms, pp. 111-12, 114-15. In this volume there are extensive quotations of Herman Bavinck. Also see Geisler, Decide for Yourself, pp. 91-102.
If God's word is heard through all of the horrible distortions through the human medium then how do we know that we are getting God's word? Do you see what I'm getting at? People can remain an athiest and think that they are a Christian by believing in nothing.
Lewis writes,"I have therefore no difficulty in accepting, say, the view of those scholars who tell us that the account of Creation in Genesis is derived from earlier Semitic stores which were Pagan and mytical.", p177, "A General Introduction to the bible" by Dr. Norman Geisler (Cited by Clyde A.S. Kilby, The Christian World of C.S. Lewis, p. 153.)
I'll save you a lot of quotes and get to the point ->
In Geisler's summary, ""Lewis believed in a fallible Bible that manifests varying degrees of inspiration. He saw a process of development whereby myth becomes history. God providentially guided the natural and errant literaary productions of the past. Then, at the appropriate moment, God adopted that natural myth and elevated it into the service of the Word of God. He now speaks through it to the edification of believers."-p. 177, "A General Introduction to the Bible" (Contemporary Theories of Revelation and Inspiration).
C.S. Lewis is reported to not hold to an Orthodox position on the Trinity that only a scholar like Dr. Norman Geisler would point out.