Protestant
Why do Catholics believe in the Real Presence in the Eucharist?
Jesus said plainly, 'This is my body' and 'my flesh is true food,' and every generation of Christians from the apostles onward understood the Eucharist as truly his Body and Blood, not a mere symbol.
Catholic teaching
At Mass, by the words of Christ spoken through the priest, the substance of bread and wine is truly changed into the Body and Blood of Christ while the appearances remain — what the Church calls transubstantiation.
Biblical foundation
'This is my body... this is my blood' at the Last Supper (Matt 26:26-28) is confirmed by John 6, where Jesus insists 'my flesh is food indeed' even as many disciples leave rather than accept it (Jn 6:53-66), and by Paul's warning against unworthy reception of the Lord's 'body and blood' (1 Cor 11:27-29).
Scripture references
Historical evidence
St. Ignatius of Antioch, a disciple of the apostles, wrote around AD 107 condemning those who deny that the Eucharist is 'the flesh of our Savior,' showing this was the universal apostolic-era belief, not a later development.
The Church Fathers
“They abstain from the Eucharist and from prayer because they do not confess that the Eucharist is the flesh of our Savior Jesus Christ.”
“We have been taught that the food which is blessed... is the flesh and blood of that incarnated Jesus.”