What does it mean when Jesus says you must eat my body and drink my blood?

The passage in question is in John 6. Here is a key passage in this chapter:

John 6:53Then 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.
See All... 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. 54 Whoso eateth my flesh, and drinketh my blood, hath eternal life; and I will raise him up at the last day. 55 For my flesh is meat indeed, and my blood is drink indeed. 56 He that eateth my flesh, and drinketh my blood, dwelleth in me, and I in him.  

First, consider what this does not mean.

1. It is not a command for the Jews to kill Jesus so they can, as cannibals, eat His flesh and drink His blood.
2. Neither does it refer to the Lord's Supper--called by Roman Catholics and some Protestants the eucharist.

Now, let's look at the reasons it does not refer to the taking of the bread and fruit of the vine in the Lord's Supper.

1. To eat the flesh of Christ and to drink His blood in a literal way would make those who did it cannibals. This would be wicked.
2. The drinking of blood is forbidden throughout the Bible. See Genesis 9:4But flesh with the life thereof, which is the blood thereof, shall ye not eat.
   See All...; Leviticus 7:26Moreover ye shall eat no manner of blood, whether it be of fowl or of beast, in any of your dwellings.
   See All...; Leviticus 17:10-14 [10] And whatsoever man there be of the house of Israel, or of the strangers that sojourn among you, that eateth any manner of blood; I will even set my face against that soul that eateth blood, and will cut him off from among his people. [11] For the life of the flesh is in the blood: and I have given it to you upon the altar to make an atonement for your souls: for it is the blood that maketh an atonement for the soul. [12] Therefore I said unto the children of Israel, No soul of you shall eat blood, neither shall any stranger that sojourneth among you eat blood. [13] And whatsoever man there be of the children of Israel, or of the strangers that sojourn among you, which hunteth and catcheth any beast or fowl that may be eaten; he shall even pour out the blood thereof, and cover it with dust. [14] For it is the life of all flesh; the blood of it is for the life thereof: therefore I said unto the children of Israel, Ye shall eat the blood of no manner of flesh: for the life of all flesh is the blood thereof: whosoever eateth it shall be cut off.
   See All...; Acts 15:28-29 [28] For it seemed good to the Holy Ghost, and to us, to lay upon you no greater burden than these necessary things; [29] That ye abstain from meats offered to idols, and from blood, and from things strangled, and from fornication: from which if ye keep yourselves, ye shall do well. Fare ye well.
   See All.... As seen in the Acts passage, even the Gentile Christians after the resurrection of Christ were to abstain from eating blood. If Christ was asking believers to eat His flesh and blood, He would be going against the clear teaching of scripture in numerous places.
3. In the passage in John 6, Christ clearly told them that He was speaking in a spiritual and not a literal sense. Verse 63 states, "It is the spirit that quickeneth; the flesh profiteth nothing: the words that I speak unto you, they are spirit, and they are life." Here, He tells them that the flesh profits nothing and that He is referring to the spirit and not to the flesh.

What the passage is saying is that we must spiritually partake of the flesh and blood of Christ in order to have eternal life. In Hebrews 3:14For we are made partakers of Christ, if we hold the beginning of our confidence stedfast unto the end;
See All..., believers are "made partakers of Christ." In Ephesians 3:6That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel:
See All..., we are "partakers of his promise in Christ by the gospel." To partake is to take of something as in eating. Here, we partake of the promise in Christ by means of the gospel. The gospel is the good news of the death, burial, and resurrection of Christ for the sake of sinners in order to provide them the offer of salvation. We partake of Christ by trusting in Him. Consider these parallel passages in John 6:   John 6:47Verily, verily, I say unto you, He that believeth on me hath everlasting life.
See All... Verily, verily, I say unto you, He that believeth on me hath everlasting life.   John 6:54Whoso eateth my flesh, and drinketh my blood, hath eternal life; and I will raise him up at the last day.
See All... Whoso eateth my flesh, and drinketh my blood, hath eternal life; and I will raise him up at the last day.   These verses are close to one another. The first states that everlasting life is obtained by believing in Jesus. The second states that eternal life (or everlasting life) is obtained by eating the flesh of Christ and drinking His blood. Therefore, to eat His flesh and to drink His blood means to believe on Him.   Salvation involves us being uniquely identified with Christ in both His death and His resurrection. It is no more difficult to think of our selves as eating and drinking the flesh and blood of Christ than it is to think of ourselves as being "baptized into his death" (Romans 6:3Know ye not, that so many of us as were baptized into Jesus Christ were baptized into his death?
See All...), "buried with him" (Romans 6:4Therefore we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life.
See All...), and "in the likeness of his resurrection" (Romans 6:5For if we have been planted together in the likeness of his death, we shall be also in the likeness of his resurrection:
See All...). In reality, we are saved by eating of His holy life and drinking His incorruptible blood (1 Peter 1:18-19 [18] Forasmuch as ye know that ye were not redeemed with corruptible things, as silver and gold, from your vain conversation received by tradition from your fathers; [19] But with the precious blood of Christ, as of a lamb without blemish and without spot:
See All...). But this is a spiritual truth and is not something that we must try to do physically. The real question is whether or not you have truly believed in the Lord Jesus Christ as your Saviour.