or the rite was instituted by Jesus Christ during the Last Supper; giving his disciples bread and wine during a Passover meal, Jesus commanded his followers to “do this in memory of me” while referring to the bread as “my body” and the cup of wine as “the new covenant in my blood”.
The change of the whole substance of bread into the substance of the Body of Christ and of the whole substance of wine into the substance of his Blood is called transubstantiation.
This change is brought about in the eucharistic prayer through the efficacy of the word of Christ and by the action of the Holy Spirit. However, the outward characteristics of bread and wine, that is the “eucharistic species”, remain unaltered.
Christian denominations differ in their understanding of whether they may celebrate the Eucharist with those with whom they are not in full communion.