Do then the bread and wine become the very body and blood of Christ?

Heidelberg Catechism (extended)

The Catechism
Method of Instruction in the Christian Religion
As the Same is Taught in the Reformed Churches and Schools

(with the Scripture references written out)

Note. This Catechism is fully based on the Scriptures. The references to Scripture are indicated in parentheses with a letter. For example, the letter (a) points to the texts (a) placed after the answer.

29 Lord's Day 

Q. 78. Do then the bread and wine become the very body and blood of Christ? 

A. Not at all: (a) but as the water in baptism is not changed into the blood of Christ, neither is the washing away of sin itself, being only the sign and confirmation thereof appointed of God; (b) so the bread in the Lord's supper is not changed into the very body of Christ; (c) though agreeably to the nature and properties of sacraments, (d) it is called the body of Christ Jesus. 

(a) Matt.26:29 But I say unto you, I will not drink henceforth of this fruit of the vine, until that day when I drink it new with you in my Father's kingdom. 

(b) Eph.5:26 That he might sanctify and cleanse it with the washing of water by the word, Tit.3:5 Not by works of righteousness which we have done, but according to his mercy he saved us, by the washing of regeneration, and renewing of the Holy Ghost; 

(c) Mark 14:24 And he said unto them, This is my blood of the new testament, which is shed for many. 1 Cor.10:16 The cup of blessing which we bless, is it not the communion of the blood of Christ? The bread which we break, is it not the communion of the body of Christ? 1 Cor.10:17 For we being many are one bread, and one body: for we are all partakers of that one bread. 1 Cor.11:26 For as often as ye eat this bread, and drink this cup, ye do shew the Lord's death till he come. 1 Cor.11:27 Wherefore whosoever shall eat this bread, and drink this cup of the Lord, unworthily, shall be guilty of the body and blood of the Lord. 1 Cor.11:28 But let a man examine himself, and so let him eat of that bread, and drink of that cup. 

(d) Gen.17:10 This is my covenant, which ye shall keep, between me and you and thy seed after thee; Every man child among you shall be circumcised. Gen.17:11 And ye shall circumcise the flesh of your foreskin; and it shall be a token of the covenant betwixt me and you. Gen.17:14 And the uncircumcised man child whose flesh of his foreskin is not circumcised, that soul shall be cut off from his people; he hath broken my covenant. Gen.17:19 And God said, Sarah thy wife shall bear thee a son indeed; and thou shalt call his name Isaac: and I will establish my covenant with him for an everlasting covenant, and with his seed after him. Exod.12:11 And thus shall ye eat it; with your loins girded, your shoes on your feet, and your staff in your hand; and ye shall eat it in haste: it is the LORD'S passover. Exod.12:13 And the blood shall be to you for a token upon the houses where ye are: and when I see the blood, I will pass over you, and the plague shall not be upon you to destroy you, when I smite the land of Egypt. Exod.12:27 That ye shall say, It is the sacrifice of the LORD'S passover, who passed over the houses of the children of Israel in Egypt, when he smote the Egyptians, and delivered our houses. And the people bowed the head and worshipped. Exod.12:43 And the LORD said unto Moses and Aaron, This is the ordinance of the passover: There shall no stranger eat thereof: Exod.12:48 And when a stranger shall sojourn with thee, and will keep the passover to the LORD, let all his males be circumcised, and then let him come near and keep it; and he shall be as one that is born in the land: for no uncircumcised person shall eat thereof. Exod.13:9 And it shall be for a sign unto thee upon thine hand, and for a memorial between thine eyes, that the LORD'S law may be in thy mouth: for with a strong hand hath the LORD brought thee out of Egypt. 1 Pet.3:21 The like figure whereunto even baptism doth also now save us (not the putting away of the filth of the flesh, but the answer of a good conscience toward God,) by the resurrection of Jesus Christ: 1 Cor.10:1 Moreover, brethren, I would not that ye should be ignorant, how that all our fathers were under the cloud, and all passed through the sea; 1 Cor.10:2 And were all baptized unto Moses in the cloud and in the sea; 1 Cor.10:3 And did all eat the same spiritual meat; 1 Cor.10:4 And did all drink the same spiritual drink: for they drank of that spiritual Rock that followed them: and that Rock was Christ. 

Q. 79. Why then doth Christ call the bread "his body", and the cup "his blood", or "the new covenant in his blood"; and Paul the "communion of body and blood of Christ"? 

A. Christ speaks thus, not without great reason, namely, not only thereby to teach us, that as bread and wine support this temporal life, so his crucified body and shed blood are the true meat and drink, whereby our souls are fed to eternal life; (a) but more especially by these visible signs and pledges to assure us, that we are as really partakers of his true body and blood by the operation of the Holy Ghost as we receive by the mouths of our bodies these holy signs in remembrance of him; (b) and that all his sufferings and obedience are as certainly ours, as if we had in our own persons suffered and made satisfaction for our sins to God. 

(a) John 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. John 6:55 For my flesh is meat indeed, and my blood is drink indeed. 

(b) 1 Cor.10:16 The cup of blessing which we bless, is it not the communion of the blood of Christ? The bread which we break, is it not the communion of the body of Christ? 1 Cor.10:17 For we being many are one bread, and one body: for we are all partakers of that one bread.

Source: http://www.ccel.org/creeds/heidelberg-cat-ext.txt