The Trinity--the affirmation that the one God, the divine being, is three-in-one--is unique in the realm of religious belief. That one God is at the same time three--Father, Son, and Holy Spirit--is a confusing concept, and yet one that less than 500 years after the founding of the Christianity that holds this belief was the focal point of big in-house fights, and outcries of "heresy" if the "correct" position was not held. Councils were held to hammer this out. So in the eyes of the early followers of Jesus, once the Church was established, this mattered. A whole lot. People got passionate--even hostile--over sorting out the "correct" position on it. The Nicene Creed and the Apostle's Creed, which we still sometimes use in worship, came from those times of sorting out and negotiating and debating and establishing that yes, God is somehow three-in-one. It didn't stop there. The split between the Orthodox and the Roman Catholic Church in 1053 was over one Latin phrase in the consideration of God as three-in-one--"filioque"--the consideration of whether the Holy Spirit proceeds from the Father AND the Son or just from the Father. The Orthodox said one thing, the Roman Catholics said the other thing, and they have been two different branches of Christianity since the mid-11th century. This stuff has mattered to people.
The Trinity is a confusing concept. I am aware of several analogies that are attempts to explain it. I will give you two, neither of which fully cover it, but that get at certain senses of how something can be uniquely three and at the same time uniquely one. One analogy gets at how the the three parts are one "ousios" (a Greek word), or one substance --the way in which "water" can be ice, liquid, or steam, depending on the temperature. But that doesn't get how they work together. The other analogy comes from a speaker at an event I attended in college, who said God as Trinity is like cherry pie. You have the cherries, you have the quasi-liquidy stuff the cherries are in, and you have the crust. Three different thing that all together make the one thing that is cherry pie. That's useful, but THAT doesn't get at how Father, Son, and Holy Spirit are one "ousios', since the three parts of the cherry pie are different substances.
What the "cherry pie" analogy gets closer to than the water analogy is the sense of the Trinity as community--that all three work together to be something, make something, "so" something. God created us so that God could be in relationship with us, and so that we could be in relationship with each other. And it follows that the two greatest commandments are about loving God and loving others. So if relationship is the key piece of what God is about in creation, and if relationship that is more than one-to-one is by definition a community, then God in self-relationship as three-in-one is already showing us what community looks like, and showing us that this is how we should live.
In a kind of pop culture kind of way (very well done, in my opinion, but still in the mainstream of pop culture), both the book and the movie "The Shack" depict God as Trinity with the relationship between God the Father (Mother, even, as this "person" is female), Jesus the Son, and the Holy Spirit. (It is another very profound statement about community that these three are, respectively, a Black woman, a white-esque man, and an Asian woman, but it's another conversation to explore that more fully). So God shows us, in God's self as three-in one, the Trinity, that we are supposed to view the world in community, and live in community.
So when the prophet Isaiah, who has to go to the people as a whole and challenge them on their failing to live in community (lots more involved than just that, but that is a key piece of why they have lost their nation) has this experience (Isaiah 6:1-8) in the presence of God (in a passage written long before the idea of the Trinity--or Jesus as the human expression of God, for that matter), when overwhelmed by the holiness of the whole thing, his reaction acknowledges his own sense of himself, but within community: "I am a man of unclean lips, and I live among a people of unclean lips". And when Jesus has his conversation at night with Nicodemus, his words also convey far more than an individualistic implication--in the most famous and probably most quoted verse in the Bible, and the one right after it that must not be separated from the most quoted one: "For God so loved the WORLD that he gave his only Son, so that EVERYONE who believes in him may not perish but may have eternal life. Indeed, God did not send the Son into the WORLD to condemn the WORLD, but in order that the WORLD might be saved through him." (john 3:16-17). I added the ALL CAPS--to emphasize the point of community.
Trinity Sunday, in its most foundational sense, is an affirmation of community.
OLD TESTAMENT Isaiah 6:1-8
1In the year that King Uzziah died, I saw the Lord sitting on a throne, high and lofty; and the hem of his robe filled the temple. 2Seraphs were in attendance above him; each had six wings: with two they covered their faces, and with two they covered their feet, and with two they flew. 3And one called to another and said:
‘Holy, holy, holy is the Lord of hosts;
the whole earth is full of his glory.’
4The pivots on the thresholds shook at the voices of those who called, and the house filled with smoke. 5And I said: ‘Woe is me! I am lost, for I am a man of unclean lips, and I live among a people of unclean lips; yet my eyes have seen the King, the Lord of hosts!’
6 Then one of the seraphs flew to me, holding a live coal that had been taken from the altar with a pair of tongs. 7The seraph touched my mouth with it and said: ‘Now that this has touched your lips, your guilt has departed and your sin is blotted out.’ 8Then I heard the voice of the Lord saying, ‘Whom shall I send, and who will go for us?’ And I said, ‘Here am I; send me!’
GOSPEL John 3:1-17
1Now there was a Pharisee named Nicodemus, a leader of the Jews.2He came to Jesus by night and said to him, ‘Rabbi, we know that you are a teacher who has come from God; for no one can do these signs that you do apart from the presence of God.’ 3Jesus answered him, ‘Very truly, I tell you, no one can see the kingdom of God without being born from above.’ 4Nicodemus said to him, ‘How can anyone be born after having grown old? Can one enter a second time into the mother’s womb and be born?’ 5Jesus answered, ‘Very truly, I tell you, no one can enter the kingdom of God without being born of water and Spirit. 6What is born of the flesh is flesh, and what is born of the Spirit is spirit. 7Do not be astonished that I said to you, “You must be born from above.” 8The wind blows where it chooses, and you hear the sound of it, but you do not know where it comes from or where it goes. So it is with everyone who is born of the Spirit.’ 9Nicodemus said to him, ‘How can these things be?’ 10Jesus answered him, ‘Are you a teacher of Israel, and yet you do not understand these things?
11‘Very truly, I tell you, we speak of what we know and testify to what we have seen; yet you do not receive our testimony. 12If I have told you about earthly things and you do not believe, how can you believe if I tell you about heavenly things? 13No one has ascended into heaven except the one who descended from heaven, the Son of Man. 14And just as Moses lifted up the serpent in the wilderness, so must the Son of Man be lifted up, 15that whoever believes in him may have eternal life.
16‘For God so loved the world that he gave his only Son, so that everyone who believes in him may not perish but may have eternal life.
17‘Indeed, God did not send the Son into the world to condemn the world, but in order that the world might be saved through him.