“You never know what you really believe until you have to defend it.”
“I hope you'll have the kind of life where what you stand for is so important that it makes some people outright hostile. You won't know how strong your beliefs really are until you have to defend them.”
“You never know how much you really believe anything until its truth or falsehood becomes a matter of life and death to you.”
“You may be able to write a novel, you may not. You will never know until you have worked very hard indeed and written at least part of it. You will never really know until you have written the whole of it and submitted it for publication.”
“You will never know what you are doing until and unless you have done it.”
“You can almost believe what you know is not true when you really have to.”