I will do some of your work for you:
The simple fact is you BELIEVE Darwin existed.
You believe this because you have read books about him.
You believe this because you have read books by him.
You believe this because someone(s) you trust told you that he existed.
Now, I will leave it to you to tell me what the criteria is for establishing historical proof. What PROOF do you have that Darwin existed, so that we might establish it as fact?
The word "belief" is a noun and the word "believe" is a transitive verb.
As I've said, ... beliefs belong in church.