In the Lord's Supper we are said to receive Christ and the benefits of His redemption to our spiritual nourishment and growth in grace. As our natural food imparts life and strength to our bodies, so this sacrament is one of the divinely appointed means to strengthen the principle of life in the soul of the believer and to confirm his faith in the promises of the gospel. By partaking of the bread and wine, the symbols of Christ's body and blood given for us, we are united to Him as our head, our life. He then works in us to will and to do of His own good pleasure. He works in us according to the laws of our nature in the production of everything that is good, so that it is from Him that all holy desires, all good counsels, and all just works proceed. It is not, therefore, we that live, but Christ that liveth in us.
What our Lord said to the apostles He says in the most impressive manner in this ordinance to every believing communicant: "This is my body, broken for you... this is my blood shed for you." These words when received by faith fill the heart with joy, confidence, gratitude, love, and devotion, so that the believer rises from the Lord's table refreshed by the infusion of a new life.
The efficacy of this sacrament, according to the Reformed doctrine, is not to be referred to any virtue in the ordinance itself, whether in its elements or actions; much less to any virtue in the administrator; nor to the mere power of the truths which it signifies; nor to the inherent divine power in the word or promise by which it is attended; nor to the real presence of the material body and blood of Christ (i.e., of the body born of the Virgin), whether by the way of transubstatination, consubstantiaition. or impanation; but only to the blessing of Christ and the working of His Spirit in them that receive the sacrament of His body and blood.
To summarize the Reformed position: The Lord's Supper is a holy ordinance instituted by Christ as a memorial of His death wherein, under the symbols of bread and wine, His body as broken and His blood as shed for the remission of sins are signified and, by the power of the Holy Ghost. sealed and applied to believers. Thereby their union with Christ and their mutual fellowship are set forth and confirmed, their faith strengthened, and their souls nourished unto eternal life.
In this sacrament Christ is present not bodily, but spiritually - not in the sense of local nearness, but of efficacious operation. His people receive Him not with the mouth, but by faith; they do not receive His flesh and blood as material particles, but His body as broken and His blood as shed. The union thus signified and effected is not a corporeal union, not a mixture of substances, but a spiritual and mystical union due to the indwelling of the Holy Spirit. The efficacy of this sacrament as a means of grace is not in the signs, nor in the service, nor in the minister, nor in the word, but in the attending influence of the Holy Ghost.
Charles Hodge (1797-1878)