Lord’s Supper (aka “Communion”)
From the text of 1 Corinthians 11:17-34 Christians are given the instructions concerning the sacrament of what has become called among Protestants as Communion or the Lord’s Supper.
For I received from the Lord that which I also delivered to you: that the Lord Jesus on the same night in which He was betrayed took bread; and when He had given thanks, He broke it and said, “Take, eat; this is My body which is broken for you; do this in remembrance of Me.” In the same manner He also took the cup after supper, saying, “This cup is the new covenant in My blood. This do, as often as you drink it, in remembrance of Me.” For as often as you eat this bread and drink this cup, you proclaim the Lord’s death till He comes. Therefore whoever eats this bread or drinks this cup of the Lord in an unworthy manner will be guilty of the body and blood of the Lord. But let a man examine himself, and so let him eat of the bread and drink of the cup. For he who eats and drinks in an unworthy manner eats and drinks judgment to himself, not discerning the Lord’s body. For this reason many are weak and sick among you, and many sleep. (1 Corinthians 11:23-34)
In general, Protestants visiting a Catholic church are prohibited from receiving Communion.
However, Protestants visiting a Catholic church who understand the Biblical teaching given by Paul would most likely neither argue nor insist upon participating in the Catholic ritual because the Protestant believer would know and understand that the Catholic belief concerning the teaching is inconsistent with the understanding and teachings of the first Disciples or with the Apostle Paul concerning this sacrament.
Basically, Catholics are taught that the elements of the sacrament, the bread and the wine, actually and mystically “turn into” the actual flesh and actual blood of Jesus Christ. Catholics refer to this “mystical” transformation of bread-into-flesh and wine-into-blood as “transubstantiation”.
Transubstantiation is, according to the teaching of the Catholic Church, the change of substance by which the bread and the wine offered in the sacrifice of the sacrament of the Eucharist during the Mass, become, in reality, the physical Body and Blood of Jesus the Christ.
According to Encyclopedia Brittanica, the doctrine of transubstantiation, elaborated by scholastic theologians from the 13th to the 15th century, was incorporated into the documents of the Council of Trent (1545–63).
According to Wikipedia, the Catholic Church teaches that the substance, or essence, of the Eucharistic offering (either bread alone, or bread and wine) is changed into both the body and blood of Christ. All that is accessible to the senses (the outward appearances – species in Latin) remains unchanged.:1413 What remains unaltered is also referred to as the “accidents” of the bread and wine, but the term “accidents” is not used in the official definition of the doctrine by the Council of Trent. The manner in which the change occurs, the Catholic Church teaches, is a mystery: “The signs of bread and wine become, in a way surpassing understanding, the Body and Blood of Christ.”:1333
The “Eucharist”, also called Holy Communion or Lord’s Supper, in Christianity, ritual commemoration of Jesus’ Last Supper with his disciples, at which (according to tradition) He gave them bread with the words, “This is my body,” and wine with the words, “This is my blood.” The story of the institution of the Eucharist by Jesus on the night before his Crucifixion is reported in four books of the New Testament (Matthew 26:26–28; Mark 14:22–24; Luke 22:17–20; and I Corinthians 11:23–25). The letters of the Apostle Paul and the Acts of the Apostles in the New Testament demonstrate that early Christians believed that this institution included a mandate to continue the celebration as an anticipation in this life of the joys of the banquet that was to come in the Kingdom of God.
Protestants believe as did the Disciples who sat with Jesus at the last supper. Jesus did NOT present His Disciples with pieces of His flesh and cups of His blood.
When Jesus broke the bread saying “this is My body broken for you..”, the Disciples did NOT see a piece of bread turn into a piece of Jesus’ flesh. And when Jesus took the cup of wine saying, “This cup is the new covenant in My blood…”, neither did the Disciples drink wine that had turned into blood.
Likewise, when Paul brought the teaching to the Gentiles, neither did Paul or the Gentile believers see the transformation of bread into flesh and wine into blood.