Excerpt from The Catholic Answer Bible (NAB)

Why Do We as Catholics Believe That Jesus Is Bodily Present in the Consecrated Host of the Eucharist?

In John 6:51-56, Jesus states repeatedly that whoever eats his flesh and drinks his blood has eternal life. He was speaking literally, and was so firm that many followers objected and left him (Jn 6:52, 60, 66). St. Paul concurs, in 1 Corinthians 11:27, where he states that those taking Communion "unworthily will have to answer for the body and blood of the Lord."

Moreover, in the Last Supper passages, nothing suggests that Jesus was speaking symbolically when he said of the bread that he held in his hands, "This is my body," and over the wine, "This is my blood" (Mt 26:26, 28; cf. Mk 14:22, 24 and Lk 24:30-31). Luke's account of the disciples' encounter with the risen Lord on the road to Emmaus also points to the truth, when the Lord takes bread, says the blessing, breaks it, and gives it to them--whereupon they recognize him, and he physically vanishes from their midst (Lk 24:30-31). Later, when they report to the apostles what they have witnessed, they tell "how he was made known to them in the breaking of the bread" (Lk 24:35). 

The Catholic Church requires people to accept its teachings in order to receive Communion, not out of a desire to exclude anyone, but in recognition that the Eucharist is, as St. Augustine wrote, a "sign of unity" and "bond of charity," and that partaking presupposes a commonly held belief (Eph 4:3-6), which sadly is not yet the case among all Christians.

RELATED SCRIPTURE -- Mt 26:26-28; Mk 14:22-24; Lk 22:19-20; Jn 6:47-66; 1 Cor 10:16.

CATECHISM OF THE CATHOLIC CHURCH -- 611; 790; 1088; 1323; 1329; 1335-1340; 1373-1377; 1382; 1390-1394; 1406; 1413; 1416; 1846; 2120.

Excerpt from The Catholic Answer Bible (NAB), copyright © 1987 and 1981, by Devore & Sons, Inc.; copyright 1970 by  Catholic Bible Publishers. All rights reserved. 
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