From the Heidleberg Catechism...

The Lord's Supper...

Question 75. How does the Lord's Supper remind and assure you that you share in the one sacrifice of Christ on the cross and in all his gifts?

Answer. In this way: Christ has commanded me and all believers to eat of this broken bread and to drink of this cup. He has thereby promised: First, as surely as I see with my eyes the bread of the Lord broken for me and the cup shared with me, so surely his body was offered and broken for me and his blood was shed for me. Second, as surely as I receive from the hand of the one who serves and actually taste the bread and the cup of the Lord which are given me as sure signs of the body and blood of Christ, so surely he feeds and nourishes my soul to everlasting life with his crucified body and shed blood.