It is the Holy Spirit working through the Word, and not a priest or minister that makes the sacrament efficacious for believers. God is the active party (not even the “rememberer”), and this is why we must see the Supper and the elements of bread and wine as gracious gifts from God–manna from heaven, as it were–given to us by God to communicate to us the realities of the blessings of the covenant of grace, through the signs instituted by God.
The Reformed understanding of the Lord’s Supper is grounded in a distinction between the “sign” and “seal” (the bread and wine) and that which is signified in the Supper (the forgiveness of sin through Christ’s shed blood, the “blood of the covenant”). There is also a sacramental union between the sign and what is signified as evident in our Lord’s words “this is my body.” This manner of speaking of the Supper comes from the words of institution given by Jesus to his disciples.
Now as they were eating, Jesus took bread, and after blessing it broke it and gave it to the disciples, and said, “Take, eat; this is my body.” And he took a cup, and when he had given thanks he gave it to them, saying, “Drink of it, all of you, for this is my blood of the covenant, which is poured out for many for the forgiveness of sins. I tell you I will not drink again of this fruit of the vine until that day when I drink it new with you in my Father’s kingdom” (Matthew 26:26-29).
When Jesus speaks of the bread as his body and the wine as his blood, we take him at his word without resorting to confusing the sign (bread and wine) with the thing signified (Christ’s body and blood). Nor should we insert words such as “this represents my body,” as in the case of those who believe that the Lord’s Supper is essentially a memorial meal and that nothing is received through partaking of the bread and wine. As Paul calls “Christ the rock” (1 Corinthians 10:4), so too, the bread is Jesus’ body, not because the sign is miraculously changed into the thing signified (as the Roman Catholic church erroneously contends in transubstantiation), but because Christ can speak of the bread (the sign) as though it were the thing signified (his body) using the language of sacraments as Jesus does when instituting the sacrament (Matthew 26:26 ff).
Following John Calvin, the Reformed have tried to keep in mind both the reality of Christ’s ascension, wherein Christ’s human nature is now in heaven awaiting his return (Acts 1:9-11), and the real presence of Christ in the sacrament (1 Corinthians 10:16-17). Although Christ’s human nature is in heaven, the believer receives all of his saving benefits, because, through faith, the Holy Spirit has united the believer here on earth to Christ in heaven.