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@imadul said,it's very sacred to us. But not just that,it's also a symbol. It's like turning the Notre Damme into a mosque or the Basilica of Saint Peter in Rome. Can you imagine that?
Erdogan built the Camlica mosque right across the Bosphorus. It's a huge mosque. And even though he built that and other mosques,he converted Agia Sofia into a mosque again. Why? For two reasons: To destroy Ataturk's legacy,because Ataturk turned Agia Sofia into a museum and that way he also defies the Kemalists. The other reason is for votes. He tries to get the vote and sympathy of the conservative muslims and and all those who want a return to the Ottoman past.
Also,Russia isn't the only Eastern Orthodox country in the world:
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Agia Sofia is a symbol for all these people. And you could say for some romantic Catholics as well.