C. S. Lewis was a profound Christian thinker. The odd thing is that by training this was not a theologian but a philologist and Medieval scholar.
In terms of his theological perspective, we can say that he was a traditional Protestant. In other words, he embraced traditional Orthodoxy. This is why so many evangelical and traditional mainline denominations claim him as their own.
The best way to get into the heart of C.S. Lewis's theology is by reading his slender volume, Mere Christianity. In it, he expounds his understanding of Christianity. This is not to say that he did not have non-tradtional elements in his thinking. For example, he is more inclusive than most conservative theologians.
We should also mention that he is an apologist. This means that he is one who defends the Christian faith or makes it understandable for people who have questions about the doctrines of Christianity.