Lewis saw it as an opportunity to put into effect one the things that was to be a hallmark of his writing. It is a 'Supposal' — suppose there are rational creatures on other planets that are unfallen? Suppose we meet them? "I like the whole interplanetary idea," he said, "as a mythology and simply wished to conquer for my own (Christian) point of view what has always hitherto been used by the opposite side." The result was his novel, Out of the Silent Planet (1938), in which the adventurers from Earth discover on Malacandra (Mars) three races of beings who have never Fallen, and are not in need of redemption because they are obedient to Maleldil (God).
When some of his readers failed to see what he was 'getting at', he concluded that "any amount of theology can now be smuggled into people's minds under cover of romance without their knowing it." Some years later The Inklings were treated to another of Lewis's 'supposals' in the Chronicles of Narnia. The Lion, the Witch and the Wardrobe was, he said, an answer to the question, "Supposing there was a world like Narnia, and supposing, like ours, it needed redemption, let us imagine what sort of Incarnation and Passion and Resurrection Christ would have there."
Source: Catholic World Report