The liquid served must be wine; because Jesus (and Paul) said so. Alternatively, it can be grape juice, because that's almost wine; close enough for government work, anyway. But no rice wine, elderberry wine, dandelion wine; only wine or juice from grapes is permitted.
The wine must be red, because, well, "This is my blood." So white wine is right out!
The bread must be bread, because, again, Jesus (and Paul) said so. Unleavened is okay, because...er, Passover. Except there's no indication the Last Supper was a Seder meal, and in fact the Seder probably showed up after 70 C.E. and the rise of rabbinic Judaism (I'm guessing here, to be honest), and so crackers are okay, or almost anything made from wheat that can be considered "bread." Leavened bread is actually acceptable, too. And preferably white (dunno why; but I've never seen anybody serve whole wheat, or rye, or pumpernickel....).
And we know this because....
Well, actually because "we've always done it this way;" that's why. Now, there's nothing wrong with this state of things; and it is certainly "holy" to some people. That is, you cannot defile the eucharist/communion/Lord's Supper with elements that are not approved for consumption at the table/altar/rail/in the pews by the blessed hand of How We've Always Done It. If you begin to suspect some pastoral cynicism, it is because every congregation has some practice/routine/ritual custom which they have "always done," and which is holier and more inviolate than the most central tenets of Christian doctrine or the most hallowed teachings of the Sermon on the Mount.
But I digress.....
We know these things are holy and right and true because we have always known them to be so. We know these things because this is one of the few sacraments all Christian (RC and Protestant) agree are actually sacred (i.e., sacramental). We also know these elements are symbolic; or transcendent; or transubstantiated. Well, there, you see, the consensus begins to break down. We don't even agree that the liquid has to be wine. At best we agree it must be derived from grapes; and that the solid portion be derived from wheat; but definitely not a sweet bread. Not even brioche; or a madeline. And certainly not a cinnamon roll!
If you're memory goes back far enough, you might begin to suspect where I'm going with this:
I announced a "Love Feast," something I understand is a practice of the Moravian church. I offered hot chocolate and cinnamon rolls on Sunday morning, as a "sort of" eucharist. (I was not ordained then, and my advisors were at great pains to be reassured I would not call this either a "meal" or "Communion," even though I said a communion prayer that morning. This may seem reasonable to someone in an episcopal tradition, such as the RC or the Anglian Communion, but in the loose UCC "traditions," this struck me as almost laughable. It still does.)
(Yes, my blog has been around long enough I can just recycle myself. Pretty much what I do anyway, but now I can use links!) I could not, when I served that "meal," call it a eucharist. Which is really funny, because Paul called it "eucharisto" not to invent a new word, but to use a (then) familiar one. Today, if it wasn't already a holiday and so a proper noun, we would call it "thanksgiving." His original interest in it, if his letter to the Corinthians is any guide (and it has been for us for nearly 2000 years) was in equality and egalitarianism, not in a severe definition of the elements:
What then follows are the famous words of institution in which Paul mentions "bread" and "wine" (or "the cup", and again depending on the translation) 9 times in the space of 6 verses. We can understand, in other words, how the emphasis came to fall on those two elements.
In giving you these instructions I come to something I cannot commend: your meetings tend to do more harm than good. To begin with, I am told that when you meet as a congregation you fall into sharply divided groups....The result is that when you meet as a congregation, it is not the Lord's Supper you eat; when it comes to eating, each of you takes his own supper, one goes hungry and another has too much to drink....Can I commend you on this point, certainly not? (1 Corinthians 11: 17-18a, 20, 22b, REB)
Unfortunately, the emphasis never fell on the verses I quoted, because it wasn't that long ago that churches still divided themselves on Sunday mornings, men on one side, women and children on the other. And before that they kicked out everybody who wasn't baptized before the eucharist could commence. And while I grew up expected to attend worship as soon as I was a toddler, it's not unusual today for children to leave worship as soon (and publicly) as possible, the better to maintain order and discipline. I was never happy with that, but I can tell you it is now "the way we've always done things," and it's not a liturgical hill that you, as a pastor, want to die on.
What was also lost in transition and translation was the idea of the eucharisto as a meal rather than as a set of elements. There isn't much evidence that Paul instituted the service as we know it, with the elements as we understand them, in every one of his churches. In fact, the evidence from this passage is that whatever was edible was brought, and Paul's complaint was, it wasn't being shared! Of course now the church is no longer home (as Paul's churches literally were) and we expect the church to play host and offer, in good hospitality, the refreshments. Er, excuse me...the elements.
You see, Paul's original emphasis was on the breakdown of social barriers, so that all would be one in Christ; neither male nor female (with the hierarchy that implied), neither slave nor free (no person the possession or possessor), and neither adult nor child (with the power differential that, too, implied). I still remember one of the best things about church when I was 13, was that I got to be treated like an adult. No where else in the world did I enjoy that privilege, even if it was, realistically, a limited one.
But we set the breakdown of social barriers aside, too, and insisted the most important part of the meal was the composition of the elements. I don't bring that up to complain about it, just to point it out. We drew boundaries and allowed a very narrow set of comestibles to be included in the Venn diagrams of what is "in" and what is "out," of what is part of the "set" and what isn't. Grape juice was an acceptable non-alcoholic substitute for wine because it was still grape juice; but no other substitute was allowed. Saltine crackers were acceptable as "Unleavened bread," but cinnamon rolls were right out!
The cinnamon rolls I served for the first time on that Christmas morning established a true eucharisto, the first one I've ever experienced (and sadly, one of the last). There was joy in that room, in a room where there should have been joy far more often (and yes, I'll take responsibility for that lacking the rest of the year). It became a tradition which I hope they still carry on. It's been years since I was there, and who can say. My point was to refresh a dying sense of joy on one of the holy days of the Christian calendar. And I've always thought it raised the question Jesus raised whenever someone in the gospels accused him of doing it wrong, of violating "We've always done it this way."
"Who do you think these rules are for?" he effectively asked. Who, indeed?