Is proof necessary

 From here. 

"I do not think there is a demonstrative proof (like Euclid) of Christianity, nor of the existence of matter, nor of the good will & honesty of my best & oldest friends. I think all three are (except perhaps the second) far more probable than the alternatives. The case for Christianity in general is well given by Chesterton; and I tried to do something in my Broadcast Talks. As to why God doesn’t make it demonstratively clear: are we sure that He is even interested in the kind of Theism which would be a compelled logical assent to a conclusive argument? Are we interested in it in personal matters? I demand from my friend a trust in my good faith which is certain without demonstrative proof. It wouldn’t be confidence at all if he waited for rigorous proof. Hang it all, the very fairy-tales embody the truth. Othello believed in Desdemona’s innocence when it was proved: but that was too late. Lear believed in Cordelia’s love when it was proved: but that was too late. ‘His praise is lost who stays till all commend.’ The magnanimity, the generosity which will trust on a reasonable probability, is required of us. But supposing one believed and was wrong after all? Why, then you would have paid the universe a compliment it doesn’t deserve. Your error would even so be more interesting & important than the reality. And yet how could that be? How could an idiotic universe have produced creatures whose mere dreams are so much stronger, better, subtler than itself?"

C. S. Lewis, letter to Sheldon Vanauken (23 December 1950), from A Severe Mercy