It has been called the most Orthodox hymn in Western hymnology, ironic, because it is one of the few Byzantine hymns borrowed by the West:
Let all mortal flesh keep silence, and with fear and trembling stand; ponder nothing earthly-minded, for the King of kings, the Lord of lords, will give to all the faithful His own self for heavenly food. Rank on rank the host of heaven spreads its vanguard on the way, the Principalities, the Authorities, the Cherubim with countless eyes, the six-winged Seraphim, veil their faces to the Presence, as with ceaseless voice they cry: Alleluia!
During the Divine Liturgy of St James, and on Great and Holy Saturday, this is the Cherubic Hymn, sung during the Great Entrance when the gifts are brought through the church, then taken back to the altar to become our Sacrifice, “on behalf of all and for all.” In the West, this deep connection with Holy Communion has been broken, and the Western hymn is sung during Nativity, and is seen as referring to Christ’s descent to Earth as Son of God, and not its original meaning of Christ’s Real Presence in Holy Communion.
While wholly compatible with the Orthodox theology of the Eucharist, or rather, the theological focus, it is not overly compatible with the Roman Catholic theological focus. It has, therefore, lost its integral connnection to Holy Communion in the West.
I say theological focus, and not theology, because while Let all mortal flesh is technically compatible with the Roman Catholic theology of Holy Communion, it is not compatible with its liturgical expression, and therefore, the elements of the theology which the Church highlights. With respect to the Eucharist, there is little theological difference between Catholic and Orthodox: We both fervently believe in the Real Presence. The only distinction is that the Orthodox don’t much care exactly how or when it happens, so have no dogma of the transubstantiation (or consubstantiation, for that matter).
The distinction, one of focus, can be seen in the Roman Catholic and Orthodox Communion Prayers (the Roman Catholic text is from the new, corrected Novus Ordo).
Lord, I am not worthy that you should enter under my roof, but only say the word and my soul shall be healed.
I believe, O Lord, and I confess that Thou art truly the Christ, the Son of the living God, Who didst come into the world to save sinners, of whom I am chief. And I believe that this is truly Thine own immaculate Body, and that this is truly Thine own precious Blood. Wherefore I pray Thee, have mercy upon me and forgive my transgressions both voluntary and involuntary, of word and of deed, of knowledge and of ignorance; and make me worthy to partake without condemnation of Thine immaculate Mysteries, unto remission of my sins and unto life everlasting.
Of Thy Mystical Supper, O Son of God, accept me today as a communicant, for I will not speak of Thy Mystery to Thine enemies, neither will I give Thee a kiss as did Judas; but like the thief will I confess Thee: Remember me, O Lord, in Thy Kingdom.
Not unto judgment nor unto condemnation be my partaking of Thy Holy Mysteries, O Lord, but unto the healing of soul and body. Amen.
The Roman Catholic prayer mentions unworthiness at the beginning, but does not mention the consequences of partaking in a state of unpreparedness. Contrast this with the Orthodox prayer, which mentions the consequences of partaking in a state of unpreparedness three times (bolded in the quotation above).
It is this heavy emphasis on the fear of God, the awe of His Majesty and the possible consequences of our approaching the Divine Mysteries in our fallen state, that distinguishes Holy Communion in Eastern Christianity from Catholic, or any Western, Communion rite. The priest calls us to approach the Chalice with:
With fear of God, with faith and love, draw ye near.
We stand in fear and trembling before the Chalice, the presence of God, terrified that to partake without full preparation will condemn us. And we pray, we beg God, not to condemn us, for we are fallen, but instead, to heal us with our partaking of His Holy Mysteries.
This focus, de-emphasized in the West, explains much in differences between Roman Catholic and Orthodox (or Eastern Rite) Communion practices. It is the Roman Catholic norm to commune if one attends Mass. This is not the case in Orthodoxy, or Eastern Rite churches. Probably 50% of my parish communes on a regular Sunday Liturgy. The influence of converts from Western Christianity is increasing the frequency of Holy Communion, but Eastern Christianity will most likely never develop the assumption that if you attend Liturgy, you are prepared to partake. The fear of God, of His Divine Presence, and approaching Him without due preparation is far too emphasized in Eastern Christianity.
This focus also explains the close connection between Confession and Holy Communion in Eastern Christianity, and partly, why practicing Orthodox go to Confession regularly and frequently compared to their Roman Catholic counterparts. It is partly by going to Confession that we prepare ourselves to approach the Chalice, and although we have no legal prescription for Confession, no more than we have Days of Obligation, the priest may withhold the Chalice if he believes the communicant is not prepared, and likely will, not to judge the communicant, but both to protect the Holy Mysteries from being profaned, and to protect the soul of the unprepared.
Even the Holy Scriptures present the first Eucharist in the context of betrayal.
For I have received of the Lord that which also I delivered unto you, that the Lord Jesus the same night in which he was betrayed took bread: And when he had given thanks, he brake it, and said, Take, eat: this is my body, which is broken for you: this do in remembrance of me.
After the same manner also he took the cup, when he had supped, saying, this cup is the new testament in my blood: this do ye, as oft as ye drink it, in remembrance of me.
For as often as ye eat this bread, and drink this cup, ye do shew the Lord’s death till he come. (1 Corinthians 11:23-26)
We do not approach the Chalice casually, but with the same fear that the Israelites regarded the Holy of Holies, and for the same reason. Our God is a God of Majesty, Power, and Awe, Christ Pantocrator, the King of the Universe, and we quake in His Holy Presence.
+Glory to the Father and to the Son and to the Holy Spirit.
We worship +the Father and his Son and the Holy Spirit, the Holy Trinity, one in Essence, and we cry out with the Seraphim:
+Holy, Holy, Holy are Thou, O Lord!