What is the one thing that Winston and Julia are unwilling to do for the Brotherhood? What s the one thing, according to Winston, that the Party does not have the power to do to them? Do you agree?

Winston and Julia refuse to betray each other. They believe that the Party can't get into their inner thoughts and feelings. Sure the Party can make them confess against each other but they feel the Party can't make them actually feel like betraying each other. This book is so dark that in its context I'm afraid the Party is able to pull this off.