What an interesting question. I never even heard of, 'Ancient of Days,' until they sang it in a church song, recently, and I had to look it up in the bible to see where the phrase came from because family was asking me to whom it referred.
I assumed God but that was based on the lyrics of the song and the scripture I referred to at the time.
In the Orthodox Church we use hymns, some of which are recent, but the average age is around a thousand years, which are extremely theologically detailed. These hymns vary between the predominant Byzantine Rite of the Eastern Orthodox Church, and the Western Rite, which is based on traditional Anglican and Roman Catholic liturgies and is used by the Antiochian and ROCOR Western Rite Vicarates, for Western Christians who are in communion with the Eastern Orthodox Church but wish to worship according to the traditional Western practice. However, you will, regardless of rite, never hear a hymn lacking in rich theological meaning. Our hymns are in this respect more doctrinal than even the better works of Charles Wesley.
For example, on Easter, our equivalent of “Christ our Lord Has Risen Today” is the Paschal Troparion:
Christ is risen from the dead,
by death trampling death,
and to those in the tombs
granting life!
This hymn manages to be both shorter than Wesley’s and at the same time contains more dogmatic information.
One hymn you will hear at every Divine Liturgy is exquisite for how it clarifies the relationship between the humanity and divinity of Christ our Lord, God and Savior, called Ho Monogenes, a hymn we share with our Oriental Orthodox brethren (the Syriac / Assyrian, Coptic, Armenian, Ethiopian, Ertirean and Indian / Malankara Orthodox), a rather good article of which can be found on this Coptic website:
O Monogenis "Only Begotten Son": The Hymn That Shaped Coptic and Byzantine Worship | St Shenouda Press: Orthodox Store
Finally, in the Eastern Orthodox church, this being Monday in the Seventh Tone* we have some lovely hymns proper to this day, for example, these Troparia sung with the Beatitudes in the Divine Liturgy in those churches still using the traditional Typikon:
Comely and good to taste was the fruit which brought death upon me. But Christ is the Tree of life, and eating thereof I die not, but cry out with the thief: Remember me in Thy kingdom, О Lord!
Like the faithful Canaanite woman I cry out in the pain of my heart: Have mercy on me, О Savior, in that Thou art good; for ever tempest-tossed I have a soul beset by all the wiles of the enemy!
The cherubim and seraphim, the thrones, principalities and powers, the archangels, the armies of angels, the dominions and most wise authorities, ever glorify Thee, О Lord our Benefactor.
Uplifted unto God, the spiritual athletes utterly cast down the wicked uprisings of the enemy; and, revealed as victors, they now live amid joy in the heavens, resplendent in incorrupt glory.
* The Eastern Orthodox and Syriac Orthodox liturgies are unique in that they alone have both a tone of the week, basically a combination of one of eight musical modes analogous to the eight modes of Gregorian chant and specific lyrics for that mode, and also a devotional or liturgical theme for each day of the week; thus there are eight sets of daily hymns. These are in addition to the hymns proper for each calendar day of the year, including the major fixed feasts like Christmas, the Baptism of Christ, the Annunciation, the Transfiguration and so on, and the major feasts and the Great Fast of Lent connected to the date of Pascha (Easter Sunday) which varies from year to year; at some times of the year these three cycles overlap producing very beautiful services. Indeed the Eastern Orthodox liturgy repeats on a 537 year cycle, meaning that if all the propers are followed, you would never hear exactly the same hymns twice, but the hymns of our principal worship service, the Divine Liturgy, are mostly fixed. There are also many different musical settings for the hymns in the Eastern Orthodox liturgy, so a Russian or Ukrainian Orthodox liturgy will usually sound very different from a Greek Orthodox or Georgian Orthodox liturgy.
