As the Small Catechism puts it, “How can water do such great things?–Answer.
It is not the water indeed that does them, but the word of God which is in and with the water, and faith, which trusts such word of God in the water. For without the word of God the water is simple water and no baptism. But with the word of God it is a baptism, that is, a gracious water of life and a washing of regeneration in the Holy Ghost, as St. Paul says, Titus, chapter three: By the washing of regeneration and renewing of the Holy Ghost,which He shed on us abundantly through Jesus Christ, our Savior, that, being justified by His grace, we should be made heirs according to the hope of eternal life. This is a faithful saying.”
And, on the Lord’s Supper: “How can bodily eating and drinking do such great things?
It is not the eating and drinking, indeed, that does them, but the words which stand here, namely: Given, and shed for you, for the remission of sins. Which words are, beside the bodily eating and drinking, as the chief thing in the Sacrament; and he that believes these words has what they say and express, namely, the forgiveness of sins.”
So it is the Word of Jesus that makes these things Sacraments (i.e., Jesus’ means to deliver forgiveness of sins, life, and salvation), and if the Word is not present, the promises of Jesus cannot be either. Usually, heterodox churches change or otherwise modify the Words of Institution at the Lord’s Supper, but they do not usually change the baptismal formula. If Jesus’ words in Matthew 28 are modified, then it would be questionable or doubtful whether that baptism is valid. Rome uses the words, and Lutherans have always confessed that the Body and Blood of Christ are present there, but the doctrine of the Sacrifice of the Mass obscures Christ’s words with human work.
In other words, it’s the words, not the individual’s faith, by which Jesus makes His Body and Blood to be present.
Hope that helps,
Pr. Timothy Winterstein