Saint Thomas would not have been helped by modern science if it had been present in his time. The question is a theological one and science doesn’t have the tools to determine theological matters. He used Aristotelian philosophy as a tool which works quite well. The substance or essence (that which makes a thing what it is) of the bread and wine changes, while the accidents or non-essentials (the appearance and all that is physically measurable) remain the same. There is no physical way of determining the change. There never was. It remains a matter of faith for us—no less than it was for the apostles.
When so many of His disciples left because they couldn’t accept the idea of consuming His flesh and blood (Jews were not allowed to consume ANY kind of blood), Jesus turned to the twelve and said: “Will you also go away? Simon Peter answered him, ‘Lord, to whom shall we go? You have the words of eternal life; and we have believed and have come to know, that you are the Holy One of God’” Jn 6: 68. The apostles didn’t understand any more than those who left understood. But they trusted Him because they loved Him. They couldn’t prove that a change would take place that couldn’t be verified by physical examination. Nor can we. The question remains, do we love Him enough to trust Him in this. For over two thousand years millions of Catholics have.
I do pray that, after considering the fact that God would allow Himself to be tortured and put to death by people He created from nothing-because of a love so beyond our minds to fathom, you will be one of them.
Fr. Vincent Serpa, O.P.