Seeing people turn away from God to the nothing is heartbreaking, if for no other reason, than for the one Lewis portrays here.

When the great moment came and the beast spoke, [uncle Andrew] missed the whole point for a rather interesting reason. When the lion had first begun singing, long ago when it was still quite dark, he had realized that the noise was a song. And he had disliked the song very much. It made him think and feel things he did not want to think and feel.

Then, when the sun rose and he saw that the singer was a lion (“only a lion,” as he said to himself) he tried his hardest to make himself believe that it wasn’t singing and never had been singing—only roaring as any lion might in a zoo in our own world.

“Of course it can’t really have been singing,” he thought, “I must have imagined it. I’ve been letting my nerves get out of order. Who ever heard of a lion singing?” And the longer and more beautifully the lion sang, the harder Uncle Andrew tried to make himself believe that he could hear nothing but roaring.

Now the trouble about trying to make yourself stupider than you really are is that you very often succeed [emphasis added]. Uncle Andrew did. He soon did hear nothing but roaring in Aslan’s song. Soon he couldn’t have heard anything else even if he had wanted to. And when at last the lion spoke and said, “Narnia awake,” he didn’t hear any words: he heard only a snarl. And when the beasts spoke in answer, he heard only barkings, growlings, bayings, and howlings. — C. S. Lewis in The Magician’s Nephew (Collier Books, 1970), pp.125-126.