Truth Himself speaks truly...

    "Do not doubt whether this is true, but rather 

receive the words of the Savior in faith, 
for since He is the truth, He cannot lie." ~ St. Cyril 

"An argument was once made use of by a pious painter named Leonardo.  He one day met in an inn two men, one of whom was a Lutheran and the other a Calvinist.  They were ridiculing the Catholic doctrine about the Blessed Sacrament.  

The Calvinist pretended that by these words, 'This is My Body,' it was only meant that the bread signifies the Body of Christ; the Lutheran, on the other hand, asserted that this was not true, but that they meant that bread and wine, in the moment of their reception, became, by the faith of the recipient, the Body and Blood of Christ. 

While this dispute was going on, Leonardo took a piece of paper and drew the image of Our Lord Jesus Christ, with Luther on the right hand and Calvin on the left.  Under the image of our Saviour, he wrote the words: 'This is My Body.' Under the figure of Calvin he wrote: 'This signifies My Body'; and under that of Luther: 'This becomes My Body in the moment that you eat it.'  

Then handing the paper to the two disputants, he said: 'Which of these three is right, our Saviour, or Calvin, or Luther?'  They were struck at the force of the argument, and ceased to scoff at the Catholic doctrine."

Excerpt from The Blessed Eucharist: Our Greatest Treasure

by Fr Michael Muller, C.S.S.R. (pg 9)
First published 1868.  Reprinted 1973, 1994.