Holy Trinity Sunday is the First Sunday after Pentecost, ushering in the season when the church hears about Jesus' ministry and then about the church's own ministry.
This day focuses on the doctrine of the triune God, a practice begun in the Western church in the 10th century. The sermon on this day is, in essence, given a theme that challenges the preacher to let the readings articulate how we are to understand the Holy Three who are also the Holy One.
None of the readings lays out in explicit terms the trinitarian divine structure, but it is inferred. The preaching on this Sunday is difficult because a doctrine is an abstraction, but also vital for the church's faith because the relations between the members of the Trinity constitute a crucial way to understand not only God but God's involvement in the life of the world.
The preacher can use the imagery of this text to draw out the elements that accompany God's presence. (In fact, this text might invite a simple, expository sermon structure.) Standing in the holy sanctuary, the narrator has a vision which tells something about the Holy One and also about the narrator. God's presence is so large, the narrator says, the hem of the Lord's robe alone fills the temple space. This is vastness. Strange but faithful creatures envelop the throne. Smoke obscures the whole scene. We are used to the images of fire and smoke, cloud and height being associated with God. It is all here. And, in comparison with that grandeur, we see ourselves, along with the narrator, as puny and inadequate. But God's power to cleanse and make whole is ready to do its work.
The vision is one of grandeur that lies outside the scope of normal human experience, and yet it is described as happening at a specific historical time, namely, when King Uzziah died in 8th century BCE, the time of the prophets Amos and Hosea. Uzziah was admired for having enhanced the kingdom's agriculture with new wells and watchtowers in the wilderness, and he built up the army. When he contracted leprosy, he had to turn his rule over to his son, Jotham. The image of the Lord of hosts is, thus, planted in history, conjoining the palpable and the unbelievable. As a doctrine, the Trinity is deeply entwined with our lives. It is not simply an abstraction!
Faced with ultimate majesty, the narrator is brought to a realization about his own lack. He has "unclean lips" which signifies his inability to stand before the Holy One. The purity codes of the ancients are foremost in his thought. He must be purified by another agent; in this case, a seraph who touches his mouth with a coal from the altar. The purification, it must be said, is not something the narrator can do for himself. Christians do not need to shy away from noticing imagistic relationships between the coal on the altar touching the lips and the bread on the table where Christ is the host taken into the mouth. The result, in both cases, is forgiveness.
As a call narrative, this is a unique text in that the narrator (the prophet), rather than being "called" by God to serve, volunteers to be the one sent. We note, however, that the offering of the self is made only after the purification has occurred. Once made clean, the narrator stands in a position from which it is acceptable to respond to the holy call.
Please note: This Isaiah 6 reading contains important liturgical language: the sanctus. The song is an acclamation from the congregation honoring the presence of the Lord. The presider has just said, "The Lord be with you... lift up your hearts... let us give thanks..." and then prayed a prayer of acknowledgment (the Preface) for what God has done in language that calls up the time of the church year.
On Holy Trinity Sunday, the Preface speaks of God's revealing "the glory of the Father, the Son, and the Holy Spirit: equal in majesty, undivided in splendor, one Lord, one God, ever to be adored..." which orients the assembly to the day and names the Trinity. In response to that naming, the people sing the words also given to the seraphs flying around the Lord's throne in this vision, "Holy, holy, holy is the Lord of hosts; the whole earth is full of his glory" (verse 3).1 By singing the seraphs' own song, the assembly is imaged as surrounding the throne of God.
This is, of course, exactly what the assembly is doing at worship: standing before, beside, or around the signs of God's grace. We honor the enormity of God's power in the language of Isaiah's vision however we sing. The singing may be accompanied by organ, orchestra, simple instruments, or we may sing with only voices, a capella which means "in the manner of the church." We may sing many different tunes. Evangelical Lutheran Worship contains at least 17 versions of this hymn -- one in each of the ten settings and as hymns #189-193, 413, and 762.
While this vision is, on the surface, at least, a prophet's call narrative, its reach moves far beyond an individual prophet. The doctrinal focus of the day -- the trinitarian construct -- serves to place the identity and energy of the Holy Three at the heart of the preacher's exegesis. In the end, the sermon will have to contend with the three legs of the trinitarian stool -- Father, Son, and Holy Spirit -- or, in Augustine's construction: Love, Beloved, and Lover. By any names, the Trinity needs all three legs lest the stool be found unable to stand.
The Isaiah reading anchors our vision of the Trinity with Isaiah's, set on the Lord of hosts and the throne of vast and awesome might.
1Otto Kaiser, Isaiah 1-12 (Philadelphia: Westminster Press, 1983), 117. Kaiser's translation is: "his glory is the fullness of the whole earth" which offers a nuanced view of the relationship between God's glory and creation itself.