St. John Chrysostom lived the 5th century in Constantinople (the seat of the Eastern Roman, or Byzantine) empire. There he rose to the position of Archbishop. John’s liturgy standardized the text and rite of the mass and that makes him one of the early church fathers. The Byzantine empire had Greek as its official language. Saints Cyril and Methodius (Byzantine Missionaries) converted the Slavs to Christianity and devised the Russian alphabet (Cyrillic), based on Greek, to transcribe Old Church Slavonic.
I spent some time this evening looking for examples of the earliest Byzantine and Russian church music and found that not so easy. Take this piece for example, sung by Sister Marie Keyrouz, a Lebanese Maronite Nun who studied musicology and anthropology at the Sorbonne, and collected ancient Greek, Syrian, and Arabic sacred music. It sounds wonderfully middle eastern, which makes sense since early Christianity started in Israel and Syria:
If you look up early Russian orthodox church music, you might find something like this:
Tchaikovsky was enthralled by such early music and while preserving the mystical or spiritually transporting feeling of these old songs, he definitely added his own Romantic flavor. However, you don’t hear much Romantic music sounding as spine-tinglingly powerful as this.