As St. Thomas Aquinas, the Great Medieval Theologian said: "I believe all that the Son of God has spoken; There is nothing truer Truth's own Word". Faith means believing without seeing, because God has declared it to be Truth. Christ is very clear in John Chapter 6.
For those who need to see to believe, there have been Eucharistic Miracles, where the Flesh and the Blood are visibly transformed. As the Church Fathers ask. He turned water into wine at Cana, yet some find it hard to believe He can turn wine into His Blood?
"In 1970-'71 and taken up again partly in 1981 there took place a scientific investigation by the most illustrious scientist Prof. Odoardo Linoli, eminent Professor in Anatomy and Pathological Histology and in Chemistry and Clinical Microscopy. He was assisted by Prof. Ruggero Bertelli of the University of Siena.
The analyses were conducted with absolute and unquestionable scientific precision and they were documented with a series of microscopic photographs.
These analyses sustained the following conclusions:
- The Flesh is real Flesh. The Blood is real Blood...
- The Flesh and the Blood have the same blood-type: AB (Blood-type identical to that which Prof. Baima Bollone uncovered in the Holy Shroud of Turin)."
Taken from:
http://www.therealpresence.org/eucharst/mir/lanciano.html