We can think about the term "The All" as an axiom. An axiom, basically, is a statement that proves itself—that describes itself—that within itself carries its own full explanation—and thereby is undeniable.

The statement The All is One is a perfect example. The All must be one or it is not the all. It is as simple as that.

And it means that there is an all-including allness to this world.

The one whole—the All—is one. And it must be one, it has no alternative.

And thus, the statement—The All is One—is an axiom in its fullest extension.