Heidelberg Catechism · 1563

Q78. Are then the bread and wine changed into the real body and blood of Christ?

No. Just as the water of baptism is not changed into the blood of Christ and is not the washing away of sins itself but is simply God's sign and pledge, so also the bread in the Lord's supper does not become the body of Christ itself, although it is called Christ's body in keeping with the nature and usage of sacraments.
Answer · Heidelberg Catechism

Scripture proofs

  1. [1]
    Ephesians 5:26

    That he might sanctify and cleanse it with the washing of water by the word,

    Titus 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;

  2. [2]
    Matthew 26:26-29

    And as they were eating, Jesus took bread, and blessed it, and brake it, and gave it to the disciples, and said, Take, eat; this is my body. And he took the cup, and gave thanks, and gave it to them, saying, Drink ye all of it; For this is my blood of the new testament, which is shed for many for the remission of sins. …

  3. [3]
    1 Corinthians 10:16-17

    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? For we being many are one bread, and one body: for we are all partakers of that one bread.

    1 Corinthians 11:26-28

    For as often as ye eat this bread, and drink this cup, ye do shew the Lord’s death till he come. 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. But let a man examine himself, and so let him eat of that bread, and drink of that cup.

  4. [4]
    Genesis 17:10-11

    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. And ye shall circumcise the flesh of your foreskin; and it shall be a token of the covenant betwixt me and you.

    Exodus 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.

    Exodus 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.

    1 Corinthians 10:3-4

    And did all eat the same spiritual meat; And did all drink the same spiritual drink: for they drank of that spiritual Rock that followed them: and that Rock was Christ.

    1 Peter 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:

Proof texts as given in the original; verse text from the King James Version.