all that vs what vs that which

Question

What’s the difference between these three usages?

1. All that glitters is not always gold.
2. What glitters is not always gold.
3. That which glitters is not always gold.

Are they interchangeable? What kind of meanings do they convey, respectively?

Answer 1

In the context of this idiom, yes, I would say they're interchangeable. They're pronouns that are defined by the rest of the sentence that follows. But I could imagine that switching out the pronoun in this manner in another sentence or context might change the meaning slightly.

Answer 2

The standard form of the expression is all that glitters is not gold. Shakespeare, in The Merchant of Venice, used glisters, a variant of the same word.

It's not necessary to include always, because in modern language we would say Not everything that glitters is gold. The meaning is that things that look shiny and precious are not always of real value.