Matt Dillahunty | Atheist Debates - Calling Christianity a myth is offensive @SansDeity | Uploaded 1 year ago | Updated 1 hour ago
Myth: "a collection of myths especially one belonging to a particular religion or cultural tradition"
'a book disucssing Jewish and Christian mythology'

More colloquially, it's used to describe beliefs that we, generally, feel are antiquated, untrue or shouldn't be believed. As Christians have the core of their identity tied to their religion, calling it a myth can be offensive.