What does it mean to say that God, Christ, and unnamed others are mythological beings?
Let's start with that vile word, mythological. The first thing we do is to shorten it to myth. Then we look it up.
My online dictionary assigns two meanings to the word. One is the most common: a myth is "a widely held but false belief or idea." The other is: "a traditional story, especially one concerning the history of a people or explaining some natural or social phenomenon, and typically involving supernatural beings or events."
Notice the difference between these two definitions. The first one immediately rings true; it's obvious. The second sounds as if it were written by a scholar. And the meanings given in the definitions to the word are also different. In ordinary language, the word refers to some common beliefs, ideas, or theories as lies. But in the language of scholarspeak, a myth is simply a story "involving supernatural beings or events"; there are no disparaging strings attached.
Because I come from the land of scholars, I'm naturally more interested in the second definition, though I take exception to the clause, "especially one concerning the history of a people or explaining some natural or social phenomenon." It appears to contain a theory of myth that I find unconvincing, and that I don't find useful for my purposes. I like the second clause, though, because I do find it useful.
At this point, I might appeal to a consensus of scholars who study myth. But there is no such consensus. Instead, there are books and academic papers written by different scholars treating the many theories of myth--theories that tell us what myth is, how it functions in society, how myths should be interpreted, etc. Important names are bandied about: Tylor, Muller, Frazer, Levy-Bruhl, Jung, Levi-Strauss, Malinowski, Eliade, Barthes. You may have heard of a few of these icons. You may even have read one or two of these cultural anthropologists, sociologists, and comparative religionists. The chances aren't good that you've written a book about one or all of them. A blog, maybe. A book, no.
In order to cut this Gordian knot, let me stipulate a definition. A myth is a story about supernatural beings, a story that provides a model for human behavior, thus giving meaning and value to life. (If you think this sounds like Eliade, give yourself a pre-grade-inflation A.)
A mythology, then, is simply a complex of myths.
That said, it makes good sense to call God and Christ mythological beings.
But I anticipate an objection. "Christ? a metaphysical being?" Yes. But Jesus is not. Jesus was, in the highest probability, an actual, living human being. "Christ" is the supernatural title given to him by some of his followers.