In The Alchemist, what does the quote: "all is one" mean?
The all is one metaphor has to do with one of the underlying themes in The Alchemist. Namely, that we all have the ability to tap into a "universal force" for understanding. This notion that the answers to our big questions have been posed before and their answers are available to all those who seek them. The answers are there because one of the author's points is that people ask these questions already knowing (whether or not it's on a subconscious level) the answers. Sort of "good lawyers only asking questions that they know the answers to."
All is one refers to the idea that all searching people are essentially searching for answers to the same question, i.e., how to unearth and find one's "personal legend." If the author's position is accepted, then we all are one "soul" or force searching in unison for answers to questions we all already know the answers to.