344. When did Christ institute the Holy Eucharist?
Christ instituted the Holy Eucharist at the Last Supper, the night before He died.
(a) About a year before the Last Supper Our Lord promised to give us the Holy Eucharist. This promise is related in the sixth chapter of the Gospel according to Saint John. The fulfillment of this promise took place at the Last Supper.
"And while they were at supper, Jesus took bread, and blessed and broke. and gave it to his disciples, and said, 'Take and eat; this is my body.' And taking a cup, he gave thanks and gave it to them, saying, 'All of you drink of this; for this is my blood of the new covenant, which is being shed for many unto the forgiveness of sins"' (Matthew 26:26-28).
347. What happened when Our Lord said: "This is My body . . . this is My blood"?
When Our Lord said, "This is My body," the entire substance of the bread was changed into His body; and when He said, "This is My blood," the entire substance of the wine was changed into His blood.
(a) Christ could not have used clearer, more explicit words than "This is My body." He did not say, "This is a sign of My body," or "This represents My body," but, "This is My body." Catholics take Christ at His word because He is the omnipotent God. On His word they know that the Holy Eucharist is the body and blood of Christ.
See also: John 6:35-51; Mark 14:22-24, Luke 22:19-20; I Corinthians 11:23-29.