Greek Credo from 10th Century Essen Sacramentary

 The Essen Sacramentary, which is dated 950-1000AD, contains, beginning on page 405 (fol. 203r) the Ordinary Chants of the Mass in Greek, but, like most other Greek Ordinaries, it is transliterated into Latin letters. 

Of course, the Credo says:

"ke is tu Pneumatu Agyon tu Kyrion ke Zopion to ek tu Patros ek poreugomenos"

[And in the Holy Ghost, the Lord and Life-Giver, from the Father Proceeding.]

Also of interest is that it gives the Eastern Cherubic Hymn to be sung for the Offertory. 

Additional info about the Essen Sacramentary can be located at the Usuarium digital liturgical manuscript collection site.