D0c,
Howdy. I thought I would address your first question, since many people have answered the second. You ask:
What makes communion symbolic?
First, I’d like to have you look at something that Augustine taught in his writings, On Christian Doctrine, Book III:
Chapter 5.-It is a Wretched Slavery Which Takes the Figurative Expressions of Scripture in a Literal Sense.
9. But the ambiguities of metaphorical words, about which I am next to speak, demand no ordinary care and diligence. In the first place, we must beware of taking a figurative expression literally. For the saying of the apostle applies in this case too: "The letter killeth, but the spirit giveth life."22 For when what is said figuratively is taken as if it were said literally, it is understood in a carnal manner. And nothing is more fittingly called the death of the soul than when that in it which raises it above the brutes, the intelligence namely, is put in subjection to the flesh by a blind adherence to the letter. For he who follows the letter takes figurative words as if they were proper, and does not carry out what is indicated by a proper word into its secondary signification; but, if he hears of the Sabbath, for example, thinks of nothing but the one day out of seven which recurs in constant succession; and when he hears of a sacrifice, does not carry his thoughts beyond the customary offerings of victims from the flock, and of the fruits of the earth. Now it is surely a miserable slavery of the soul to take signs for things, and to be unable to lift the eye of the mind above what is corporeal and created, that it may drink in eternal light.
Chapter 10.-How We are to Discern Whether a Phrase is Figurative.
14. But in addition to the foregoing rule, which guards us against taking a metaphorical form of speech as if it were literal, we must also pay heed to that which tells us not to take a literal form of speech as if it were figurative. In the first place, then, we must show the way to find out whether a phrase is literal or figurative. And the way is certainly as follows: Whatever there is in the word of God that cannot, when taken literally, be referred either to purity of life or soundness of doctrine, you may set down as figurative. Purity of life has reference to the love of God and one's neighbor; soundness of doctrine to the knowledge of God and one's neighbor. Every man, moreover, has hope in his own conscience, so far as he perceives that he has attained to the love and knowledge of God and his neighbor. Now all these matters have been spoken of in the first book.
Chapter 16.-Rule for Interpreting Commands and Prohibitions.
24. If the sentence is one of command, either forbidding a crime or vice, or enjoining an act of prudence or benevolence, it is not figurative. If, however, it seems to enjoin a crime or vice, or to forbid an act of prudence or benevolence, it is figurative. "Except ye eat the flesh of the Son of man," says Christ, "and drink His blood, ye have no life in you."35 This seems to enjoin a crime or a vice; it is therefore a figure, enjoining that we should have a share in the sufferings of our Lord, and that we should retain a sweet and profitable memory of the fact that His flesh was wounded and crucified for us. Scripture says: "If thine enemy hunger, feed him; if he thirst, give him drink;" and this is beyond doubt a command to do a kindness. But in what follows, "for in so doing thou shall heap coals of fire on his head,"36 one would think a deed of malevolence was enjoined. Do not doubt, then, that the expression is figurative; and, while it is possible to interpret it in two ways, one pointing to the doing of an injury, the other to a display of superiority, let charity on the contrary call you back to benevolence, and interpret the coals of fire as the burning groans of penitence by which a man's pride is cured who bewails that he has been the enemy of one who came to his assistance in distress. In the same way, when our Lord says, "He who loveth his life shall lose it,"37 we are not to think that He forbids the prudence with which it is a man's duty to care for his life, but that He says in a figurative sense, "Let him lose his life"-that is, let him destroy and lose that perverted and unnatural use which he now makes of his life, and through which his desires are fixed on temporal things so that he gives no heed to eternal. It is written: "Give to the godly man, and help not a sinner."38 The latter clause of this sentence seems to forbid benevolence; for it says, "help not a sinner." Understand, therefore, that "sinner" is put figuratively for sin, so that it is his sin you are not to help.
I don’t know how much clearer it can be made than by those words for St. Augustine. Now, I’m going to address this question from a scriptural approach as you requested. Our first indication that communion is symbolic is:
John 6:63, It is the spirit that quickeneth; the flesh profiteth nothing: the words that I speak unto you, [they] are spirit, and [they] are life.
Christ is clearly clearing up the misunderstanding of those who are questioning. I state this with confidence, because even before I read Augustine, whom I quoted above, I could rationalize, myself, that nowhere in Scripture is “eating flesh and drinking blood” condoned. In fact, eating meat with blood still in it was prohibited, and eating human flesh was not permitted at all. Notice in John 6:63 above Jesus says, “ . . . the words I speak . . . are spirit . . . are life.” This is important because on other occasions prophets/teachers were commanded to eat “words” or written texts and then to go and spread/prophesy that they had eaten/consumed. Lets look at a couple of OT passages and a passage from Revelation:
Jeremiah 15:16: “Your words were found and I ate them, and your words became for me a joy and the delight of my heart.”
Ezekiel 2:8 But thou, son of man, hear what I say unto thee; Be not thou rebellious like that rebellious house: open thy mouth, and eat that I give thee
Ezekiel 2:9 And when I looked, behold, an hand [was] sent unto me; and, lo, a roll of a book [was] therein;
Ezekiel 2:10 And he spread it before me; and it [was] written within and without: and [there was] written therein lamentations, and mourning, and woe.
Ezekiel 3:1 Moreover he said unto me, Son of man, eat that thou findest; eat this roll, and go speak unto the house of Israel
Ezekiel 3:2 So I opened my mouth, and he caused me to eat that roll.
Ezekiel 3:3 And he said unto me, Son of man, cause thy belly to eat, and fill thy bowels with this roll that I give thee. Then did I eat [it]; and it was in my mouth as honey for sweetness.
Revelation 10:9 And I went unto the angel, and said unto him, Give me the little book. And he said unto me, Take [it], and eat it up; and it shall make thy belly bitter, but it shall be in thy mouth sweet as honey.
Revelation 10:10 And I took the little book out of the angel's hand, and ate it up; and it was in my mouth sweet as honey: and as soon as I had eaten it, my belly was bitter.
Revelation 10:11 And he said unto me, Thou must prophesy again before many peoples, and nations, and tongues, and kings.
So, clearly, and on more than one occasion, eating or consuming “words” signifies being spiritually enlightened or fed. In John 6, the commands to “eat and drink” were metaphorical representations of “come and believe.”
Jhn 6:27 Labour not for the meat which perisheth, but for that meat which endureth unto everlasting life, which the Son of man shall give unto you: for him hath God the Father sealed.
Jhn 6:35 And Jesus said unto them, I am the bread of life: he that cometh to me shall never hunger; and he that believeth on me shall never thirst.
Jhn 6:37 All that the Father giveth me shall come to me; and him that cometh to me I will in no wise cast out.
Jhn 6:40 And this is the will of him that sent me, that every one which seeth the Son, and believeth on him, may have everlasting life: and I will raise him up at the last day.
Jhn 6:47 Verily, verily, I say unto you, He that believeth on me hath everlasting life.
Then:
Jhn 6:50 This is the bread which cometh down from heaven, that a man may eat thereof, and not die.
Jhn 6:51 I am the living bread which came down from heaven: if any man eat of this bread, he shall live for ever: and the bread that I will give is my flesh, which I will give for the life of the world.
Jhn 6:53 Then Jesus said unto them, Verily, verily, I say unto you, Except ye eat the flesh of the Son of man, and drink his blood, ye have no life in you.
Jhn 6:54 Whoso eateth my flesh, and drinketh my blood, hath eternal life; and I will raise him up at the last day.
Jhn 6:55 For my flesh is meat indeed, and my blood is drink indeed.
Jhn 6:56 He that eateth my flesh, and drinketh my blood, dwelleth in me, and I in him.
So, you have Christ saying:
1) Jhn 6:35 And Jesus said unto them, I am the bread of life: he that cometh to me shall never hunger; and he that believeth on me shall never thirst.
2) Jhn 6:47 Verily, verily, I say unto you, He that believeth on me hath everlasting life.
And
3) Jhn 6:51 I am the living bread which came down from heaven: if any man eat of this bread, he shall live for ever: and the bread that I will give is my flesh, which I will give for the life of the world.
4) Jhn 6:54 Whoso eateth my flesh, and drinketh my blood, hath eternal life; and I will raise him up at the last day.
Now, one will look at these passages and come to one of two conclusions:
1) They contradict each other, therefore Christ is contradicting Himself.
Or
2) “Eating and drinking” symbolizes “coming and believing.”
Now, we know Christ didn’t contradict Himself, so we are left with one answer.
Eating and drinking are symbolic of coming to and believing in Christ.
Now look at John 7, it ties this truth up nicely:
Look at this verse:
John 7: 37-38 “If anyone is thirsty, let him come to me and drink. He who believes in me, as Scripture said, ‘From his innermost being will flow rivers of living water.’”