Transubstantiation is a doctrine of the Roman Catholic Church. The Catechism of the Catholic Church defines this doctrine in section 1376.
“The Council of Trent summarizes the Catholic faith by declaring: ‘Because Christ our Redeemer said that it was truly his body that he was offering under the species of bread, it has always been the conviction of the Church of God, and this holy Council now declares again, that by the consecration of the bread and wine there takes place a change of the whole substance of the bread into the substance of the body of Christ our Lord and of the whole substance of the wine into the substance of his blood. This change the holy Catholic Church has fittingly and properly called transubstantiation.”
In other words, Catholic Church teaches that once an ordained priest blesses the bread of the Lord’s Supper, it is transformed into the actual flesh of Christ (though it retains the appearance, odor, and taste of bread); and when he blesses the wine, it is transformed into the actual blood of Christ (though it retains the appearance, odor, and taste of wine).
Those who believe that in the Eucharist the bread and wine become instead the body and blood of Christ see this as indicated in the New Testament, in the Eucharistic discourse given by Christ in John 6, and in 1 Corinthians 10-11, where Saint Paul equates the body and blood of Jesus with the “bread” and “cup of benediction” used in the Eucharist.
Eastern Orthodox, Oriental Orthodox, and Roman Catholics, who together constitute the majority of Christians, hold that the consecrated elements in a valid celebration of the Eucharist indeed become the body and blood of Christ. This belief is held also by some Reform and Protestant Christian churches, Lutherans and Anglicans, though they generally deny transubstantiation.
While there is a large body of theology noting the many Scriptural supports for transubstantiation, in general, Orthodox and Catholics consider it unnecessary to "prove" from texts of Scripture a belief that they see as held by Christians without interruption from the earliest, apostolic times. They point out that the Church and its teaching existed before it assembled and canonized the New Testament, and even before any individual part of the New Testament was written, They also point out that early Christians such as Ignatius of Antioch, Justin Martyr, and Clement of Rome (who were much closer to the event than those who have later proposed a figurative interpretation of the Eucharist), described the Eucharist as truly the body and blood of Christ. They see nothing in Scripture that in any way contradicts this age-old Christian belief that the reality beneath the visible signs in the Eucharist is the body and blood of Christ and no longer bread and wine. Instead, they see this teaching as the same teaching in the Bible's reports of what Christ himself and Paul the Apostle taught.