There may be older Christian hymns, but the oldest hymn accompanied with musical notation is surely the fragmentary one found at Oxyrhynchus:
The Ancient Peoples blog provided a transcription and translation of the text:
[?ฯฯ]ฯ ฯฮฑฮฝฮทฯ ฯฮนฮณฮฑฯฯ ฮผฮทฮดแพฝ ฮฑฯฯฯฮฑ ฯฮฑฮตฯฯฮฟฯฮฑ ฮป[ฮตฮนฯ]ฮต
[ฯ]ฮธฮฟฮฝ[.].ฮปฮตฮน[โฆ]ฯ[โฆ]ฯฮฟฯฮฑฮผฯฮฝ ฯฮฟฮธฮนฯฮฝ ฯฮฑฯฮฑฮน ฯ ฮผฮฝฮฟฯ ฮฝ ฯฯฮฝ ฮดแพฝ ฮทฮผฯฮฝ
[ฯ]ฮฑฯฮตฯฮฑ ฮงแพฝ ฯ ฮนฮฟฮฝ ฮงแพฝ ฮฑฮณฮนฮฟฮฝ ฯฮฝฮตฯ ฮผฮฑ ฯฮฑฯฮฑฮน ฮดฯ ฮฝฮฑฮผฮตฮนฯ ฮตฯฮนฯฮฟฯฮฝฮฟฯ ฮฝฯฮฟฮฝ ฮฑฮผฮทฮฝ ฮฑฮผฮทฮฝ ฮบฯฮฑฯฮฟฯ ฮฑฮนฮฝฮฟฯ
[โฆ]ฮด[ฯฯ]ฮท[ฯฮน] ฮผฮฟฮฝฯ ฯฮฑฮฝฯฯฮฝ ฮฑฮณฮฑฮธฮฟฮฝ ฮฑฮผฮทฮฝ ฮฑฮผฮทฮฝ
These lyrics have been translated as:
Let the Luminous stars not shine,
Let the winds (?) and all the noisy rivers die down;
And as we hymn the Father, the Son and the Holy Spirit,
Let all the powers add โAmen Amenโ
Empire, praise always, and glory to God,
The sole giver of good things, Amen Amen.
Seeย the booklet that comes with the CD with a recording of the hymn, Musique de la Grece Antique (Ancient Greek Music), for more information.