Comment by rmrfchik

Slavic had a number of alphabets. They drastically differs, compare Glagolitic and Cyrillic scripts. Now we (I'm Russian) in Russian use Cyrillic script which was heavily modified since invention. Seems like Onfim use Cyrillic script but many letters was made obsolete (like Yus). So, Onfim speak Slavic language and use Cyrillic script.