Eastern Orthodoxy was the primary religion of the Byzantine
Empire (330-1453)
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The majority religion in the Ukraine is Eastern Orthodoxy.
Eastern Orthodoxy is an old, very traditional form of Christianity
which is mainly practiced in Russia, Greece, and Cyprus.
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Eastern Orthodoxy is a Christian monotheistic religion and
worships the God of the Old and New Testaments in the Bible.
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WikiAnswers has a lot of information on Eastern Orthodoxy. If you go to the main page, click on "Religion & Spirituality" questions, then click on "Christianity" and you will find a heading called "Eastern Orthodoxy" which has many questions and answers on this topic.