The Roman Catholic Church teaches that in the Eucharistic offering bread and wine are changed into the body and blood of Christ. The reaffirmation of this doctrine was expressed, using the word "transubstantiate", by the Fourth Council of the Lateran in 1215. It was later challenged by various 14th century reformers—John Wycliffe in particular.
The manner in which the change occurs, the Roman 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 precise terminology to be used to refer to the nature of the Eucharist and its theological implications has a contentious history, especially in the Protestant Reformation.
In the Greek Orthodox Church, the doctrine has been discussed under the term of metousiosis, coined as a direct loan-translation of transsubstantiatio in the 17th century. In Eastern Orthodoxy in general, the Sacred Mystery (Sacrament) of the Eucharist is more commonly discussed using alternative terms such as "trans-elementation" (μεταστοιχείωσις, metastoicheiosis), "re-ordination" (μεταρρύθμισις, metarrhythmisis), or simply "change" (μεταβολή, metabole).
The belief that the bread and wine that form the matter of the Eucharist become the body and blood of Christ appears to have been widespread from an early date, with early Christian writers referring to them as his body and the blood. They speak of them as the same flesh and blood which suffered and died on the cross.
The short document known as the Teaching of the Apostles or Didache, which may be the earliest Christian document outside of the New Testament to speak of the Eucharist, says, "Let no one eat or drink of your Eucharist, unless they have been baptized into the name of the Lord; for concerning this also the Lord has said, 'Give not that which is holy to the dogs'."