If I said that I'm an atheist in your nation?

[A m e n r i a]

A nearby ashigaru would walk up to you and take you to the nearest mental hospital politely and gently.

[Romanum Dominium]

In the Romanum Dominium, participation in the Imperial Cult is mandatory for except for practicing, ethnic Jews (and sometimes Christians). You don't have to believe in the gods but you do have to (publicly) worship them and make a sacrifice at least once a year (or pay a priest to do it for you).

Feel free to say that the gods aren't real. Feel free to say that you're worshipping under protest. Don't refuse to worship and don't try to get other to not worship.

[Stellar Colonies]

Nobody would really care.

[Risen Prussia]

My country is super spiritual so you would have a high chance of people around you being a buddy and helping you out. You would have no political disadvantages.

[Salirosa]

If you're extremely vocal about it and are spreading atheist literature you may be locked up for attempting to subvert Salirosan society.

[Groblicandia]

You would be Beheaded

[The Xenopolis Confederation]

People would spam shrekdora memes at you, even though the majority of them are non-religious themselves.

[Gowinia]

Nobody would care that much, but if you said that to a Shrekist he would say :
« This is my swamp. »

[Tigeria]

No one would bat an eye, your beliefs are yours, however if you intend to restrict religious liberty, you'd be throwing yourself in a tiger pit.

[Hvitmannshjem]

The government wouldn't do anything, but you'd get plenty of dirty looks from the citizens, as Hvitmannshjem is strongly adherent to Eastern Orthodoxy.

[Katsur]

Aside from select minority groups no one would bather an eye

[Drescauld]

Most citizens wouldn’t care, as many would agree with you besides religious radicals.

[Persagonia]

In Urban areas: They wouldn't care, they've been used to secularist laws
In Rural areas: They would be shocked but then, they would proceed to invite you for coffee and a dance just like any offering to visitors

[Egyptian Pharocracy]

We'd probably shrug and nod and go on about our business, not really giving a damn.

In a nation where the Avatars of the Egyptian Pantheons walk the streets regularly among the citizens, most Kemeticists and Atenists would probably take you for a Flat Earth Atheist or a Hollywood Atheist, and most Coptics and Zoroastrians wouldn't care.

In both cases, most people would wonder why you were making a big deal of making a statement about it: roughly 6.4% of our adult population is atheist or agnostic, and it's just not that unusual.

[Alizeria]

There would be a lot of eye-rolling because it's considered deeply impolite to discuss religion publicly. You'd also possibly become the target for mockery.

Most people though wouldn't really care, though in some rural areas you might arouse suspicion.

[Tropical Isles]

even though a large portion of our population is religious, nobody would care.

[Siberai]

You would be treated like ny normal citizen. No rights, always watched, at risk of execution.

[Pokemon Trainer Red]

Nobody would really care, but followers of Lord Helix have been known to look down on atheists (or merely anybody who doesn't believe in Lord Helix).

[Vegaslovakia]

Vegaslovakia is an irreligious society. So, in whatever the local dialect was, another citizen would probably say something like, "Well, yeah..."

[Ammaroth]

It really wouldn't matter in the slightest. Religion doesn't play an important role in the everyday life of most Ammarothi citizens, even those who actively describe themselves as religious.

[Remulia]

who cares?

[Aguipinto]

Good for you!

[-Jochistan-]

People wouldn't mind. Most of Jochistan's population is quite religious, but atheists aren't unheard of

[Aikoland]

No one would mind, around 29% of the population of Aikoland reports being atheist (and that's included within the 57% who are merely 'not religious', for the record), so it's not like it's uncommon or unheard of.

[New Pyrrhius]

there is nothing left to hide from