This beautiful hymn, Phos Hilaron (Gentle Light (Svete Tikhy in Russian; Lumen Hilare in Latin; O Gladsome Light in English) is the first church hymn in its proper sense. Every evening when the Christians gathered for the service they sang it and lit the candle or lamp that symbolised the ever-living light of Jesus. You can read the story of the hymn. Below is a video of its performance in Old Church Slavonic by the monks of the Valaam Monastery.
(The evening service is on the way in Russian churches, but I have to visit the library today to take back the books).
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