This or That? When to Use Each

This is a bit irrelevant, but I have a question which pertains to rhetoric. When writing, does one use "this" or "that," or does it not matter?

For example, I have written a statement in one sentence. I make reference to the statement in the next sentence. Which of the two should I use, this or that?

ex - "Math is good. This is true because..."
or "Math is good. That is true becuase..."