This question already has an answer here:
Given a context-free grammar and a regular grammar, check whether they are equivalent. It's a fact that it's undecidable, but how could I prove it?
I want to clarify that my question is not about determining whether a CFG describes a regular language. In my problem I am given a CFG and a regular language and I need to tell whether they are equal or not. These are different problems as far as I understand, mine is easier than that one.