Most Greek writers are by default Orthodox, although that may be in little more than form. However, that doesn't mean they write 'Orthodox fiction'. Most such fiction is a) geared towards children, and b) not very good, as it's essentially catechism with characters, rather than a solid literary work.
One author for whom Orthodoxy was a central theme and
is likely to be found translated in English is Alexandros Papadiamantis. (He's also a favourite of mine, so I'm biased.