ANKARA, June 21 (Reuters) - Turkish President Tayyip Erdogan ordered officials on Sunday to resume talks on reopening an Orthodox Christian seminary near Istanbul, an issue raised by U.S. President Donald Trump, who is expected to visit Ankara next month for a NATO summit. The Halki seminary, founded in 1844 and shut by the Turkish state in 1971, played a central role in the Eastern Orthodox Church as the Ecumenical Patriarchate's main theological school.