What is the correct and why to say?

It was a great idea.
or
It is a great idea.

Which one is the correct and why , I’m trying to enhance my writing.
Thank you all : )
Please if you remark anything about my writing let me know.

I think both r correct ,but it depends …
“It was a great idea” means that that idea had been suggested in the past.
"It is a great idea " means that the idea is just suggested and you approve the idea .

e.g.1- A. why didn’t you like his idea?!!
B.It was great but it was impossible to be achieved.

e.g.2- A. what about going to Hollywood cinema !!
B. that’s a great idea .
I’m not really sure ,but that’s what I think …

I support Ola.fun5’s opinion. Both variants are possible and their usage depends on the context and the time when the mentioned idea was proposed.

Thank you so much

I’m Manushak. My short name is Mane
I’m 26 years old.
I’m from Armenia.
I’m databases and reporting manager.
I’m not married yet.
I’m not tall.
I’m interested in learning English.

good day everybody,