[What do you think of
Athanasius?]
I like it but vastly prefer (and would use)
Athanasios which is ethnically Greek so I am not suggesting you use it over
Athanasius. I just prefer it.
[If it's used as a first name, would
Nathan (like
Ned to
Edward or
Nell to
Eleanor) be a workable nickname?]
No, it's a stretch. I prefer Athan and it doesn't rhyme with
Nathan either so it seems like even more of a stretch.
[If it weren't a first name, would it be a good middle name for
Charles?]
Yes,
Charles Athanasius is quite handsome.