Beneath the surface, blithe Athenian is an unbeliever (7)

I believe the answer is:

heathen

I'm a little stuck... Click here to teach me more about this clue!

'unbeliever' is the definition.
(I've seen this before)

'beneath the surface blithe athenian is an' is the wordplay.
'HEATHEN' is hidden within the letters.
However, I'm uncertain how this is indicated.

Can you help me to learn more?

(Other definitions for heathen that I've seen before include "Barbaric" , "Uncultured" , "Infidel, pagan" , "Non-believer" , "Person lacking culture or moral principles" .)

I've seen this clue in The Irish Times.

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