How about the whole concept of gods.
Concepts are easy. I have the concept of as many gods as I could wish, one by one or together. They can look like the bearded old man of Renaissance paintings, or doves, suns, Ganesha, or Marvel comics, or Greek statuary, or Russian icons, or spirits in a No play, or illustrations in a book of fairy tales, or paintings on an Egyptian tomb, on and on. (The apophatic god is more tricky, perhaps a wisp of incense or a curtain stirred by a breeze.)
But real ones? No.
A parallel problem is that there's no concept of real "godness", the quality that a real god would have and a real superscientist who could create universes, travel in time, raise the dead &c, would lack.
If God is indeed real, does have objective existence, then that's just weird,
Instead, as I mentioned, when it comes to real gods, not only does no one know what they're actually talking about, what real entity the word "God" is intended to denote, but they don't even have the beginnings of a vocabulary appropriate for such a being,