"People who were not Christians themselves helped me to Christianity."

All My Road before Me: The Diary of C. S. Lewis
Permission Granted by Copyright Owner for Use on Book DrumAll My Road before Me: The Diary of C. S. Lewis - Credit: Photo by Steven Skaggs.
Lewis may have had in mind here an event he recorded in his account of his conversion to Christianity, Surprised by Joy:

Early in 1926 the hardest boiled of all the atheists I ever knew sat in my room on the other side of the fire and remarked that the evidence for the historicity of the Gospels was really surprisingly good. “Rum thing,” he went on. “All that stuff of Frazer’s about the Dying God. Rum thing. It almost looks as if it had really happened once.” To understand the shattering impact of it, you would need to know the man (who has certainly never since shown any interest in Christianity). If he, the cynic of cynics, the toughest of the toughs, were not—as I would still have put it—“safe,” where could I turn? Was there then no escape? (223-24)

This "hard-boiled atheist" may have been Thomas Dewar Weldon. In All My Road Before Me, 379-80, Lewis’s diary entry for Tuesday, 27 April, 1926, recounts an evening Lewis spent with Weldon. It reads in part as follows: “We somehow got on the historical truth of the Gospels, and agreed that there was a lot that could not be explained away. He believes in the Hegelian doctrine of the Trinity and said the whole thing fitted in: in fact he is a Christian ‘of a sort’. I should never have suspected it. . . . Got to bed v. late at last with a headache, regretting a wasted, tho’ interesting evening.”