Sorry, but that is like saying that without Tolstoy there would have been no Russian literature.
F. Anstey, E. Nesbit, Frank Baum, Lovecraft, Howard, Clark Ashton Smith, Ernest Bramah, Dunsany. Branch Cabell, John Collier were all already publishing (several in fact had completed their careers and died) before Tolkien and C.S. Lewis and Lloyd Alexander more or less contemporaneously. Tolkien was accessible and created a monumental fictional universe, but he was neither the only talented nor the most imaginative fantasy writer. The same with Asimov -possibly the most accessible SF writer but nowhere near the best or the most imaginative -what about Poul Anderson, Clifford Simak, Robert Silverberg, Henry Kuttner, C.M. Kornbluth, Katharine Maclean, John Wyndham, Olaf Stapledon?
I am not sure that he was ever a full-blown atheist, just very heavily riven with doubt in the early 1920s. Agree that Tolkien's influence was a major reaffirmation of his early faith and helped him back to belief but Lewis was a man very strongly influenced by Christianity with a strong nuanced understanding/appreciation of the same. I suspect that even without Tolkien, he would have eventually returned to the fold. Probably we would be crediting Charles Williams, Stoddart Kennedy or Father Ronnie Knox instead of Tolkien but Lewis was a man who wanted to believe and insightful enough not to accept Communism or Fascism as a substitute faith.