CS Lewis' That Hideous Strength is possibly, in spite -- or precisely because -- of its flaws, his best novel. It was first published in 1945, but written in 1943 just after the Battle of Stalingrad, when the Second World War was not officially over, but Lewis already takes it for granted that the Nazis have been defeated . . . or not? In fact, to paraphrase a famous sentence, Germania capta ferum victorem cepit: The post-war world, in England itself, is described as the fulfillment of Hitler's wildest dreams, with its inhuman technology and 'scientific' medicine, joined with black magic and a longing for destruction. Only apparently a paradox, actually a prophetical insight into our current society controlled by multinational corporations that do behave like that. (Quite odd to say this in a blog provided by one of them for free.)
The third part in Lewis' Space Trilogy, That Hideous Strength is quite different from the previous two episodes, as well as much longer. Like in Out of the Silent Planet and Perelandra, the main narrative trait is a brilliant mix of fantasy and science fiction; but, unlike them, THS is full of extra materials that are not strictly needed in the plot. But after all, the whole story hinges on a laboratory, so, not inconsistently, is itself a "laboratory."
As for the Christian message, it is even clearer and stronger than in most or all of Lewis' other novels. The "moral preaching" proves sometimes a bit heavy, especially in the mouth of Ransom, whose personality has become overloaded with powers of any kind and somewhat unpleasant, as it is the doom of many "goodies" in literature. But there also appear some very interesting reprocessings of episodes from John Bunyan's Pilgrim's Progress, and especially, the Question is led to its crucial point: Are you with the crucified Christ or against him?