The following is taken from Clark, p. 622
Just as he dotted the i's and crossed the t's of his scientific beliefs during the last year or so of his life, so did he recapitulate his religious convictions. To Dr. Douglas he stated: If I were not a Jew I would be a Quaker. And in an interview with Professor William Hermanns, he said: I cannot accept any concept of God based on the fear of life or the fear of death or blind faith. I cannot prove to you that there is no personal God, but if I were to speak of him I would be a liar.
As to what one could believe in, the answer was simple enough. I believe in the brotherhood of man and the uniqueness of the individual. But if you ask me to prove what I believe, I can't. You know them to be true but you could spend a whole lifetime without being able to prove them. The mind can proceed only so far upon what it knows and can prove. There comes a point where the mind takes a higher plane of knowledge, but can never prove how it got there. All great discoveries have involved such a leap.