The Real Presence...

Like the Resurrection, the Real Presence matters only insofar as it is a thing "out there" and not merely a psychological projection originating in the mind of the believer. In other words, the Eucharist is either changed or it is not, just as Christ is either risen or He is not. And if the Eucharistic change is a fact, then it is a fact not just in our hearts but even for those who disbelieve it. Similarly, if the Eucharist is not the Body and Blood, then no amount of self-willed "faith" on my part could make it otherwise. Thus, much as I would have liked to , I realized I could not rush off to my local Evangelical church and "consecrate in my heart" the unconsecrated bread and grape juice I received there anymore than I could privately write my own Scriptures or declare myself an Apostle.

For the change in the bread and wine can only be accomplished by the power and authority of the Lord Jesus Christ Himself. And this authority, according to Scripture and Catholic teaching, has been delegated by Christ to the Church solely through his Apostles. As Justin the Martyr writes: "The apostles in the memoirs, which are called the Gospels, have handed down what Jesus had ordered them to do; that He took bread and, after giving thanks, said "Do this in remembrance of me, this is my Body." In like manner He took also the Chalice, gave thanks, and said, "This is my Blood" And to them only did he give it. (First Apology)
Mark Shea
This is My Body
An Evangelical Discovers the Real Presence