When something (a thing or an idea) is mentioned the first time, we use "this" or "that". And after that, no matter how often we refer to it, we only use "it", am I right?
For example, I made two examples:
A: I received a scholarship for the graduate program.
B: "That" is one of the most surprising things I've heard today.
C: Congrats. I will tell "it" to our friends.
D: "It" really motivated me. B: Yeah I couldn't believe "it" at first."
A: I received a scholarship. "That" is one of the surprising things today. I did not believe "it" when I was told about "it". However, "it" is something real.