Is it okay to say "that which"?
I know that there are certain times to use "that" (for restricting the noun) and certain times to use "which" (for adding information).
How about "that which"?:
Truth is that which conforms to reality.
"That" is here a demonstrative pronoun, but it seems that this usage of which is incorrect.
Is this okay?
If it's not correct, how could I restate that sentence?
Yes, it is absolutely OK to use 'that which' in that way.
Reference: My favourite quote:
That which does not kill us makes us stronger
Friedrich Nietzsche