“Issue 3 - The Biblical Basis for the Catholic teaching concerning the Real Presence in the Holy Eucharist.”
The claim of core doctrine in question is outright full transubstantiation - conversion of bread and wine into literal flesh and blood. We are blessed with objective senses, and all plainly know this does not happen - ergo the doctrine is absurd. A spiritual conversion, perhaps; metaphor, at least; physical change, no.
To understand transubstantiation—as far as it can be understood—you need to brush up on Aristotelian philosophy and Thomistic theology, having to do with substance and accidents.
Obviously it is miraculous, but God is capable of changing the true inner substance while maintaining the accidental outward appearance.
If Jesus’s words, “Unless you eat my flesh and drink my blood, you shall not have life within you” were merely metaphorical, then it is hard to explain why many of his disciples walked away and left him when they heard that. They took it literally. And if that was a mistake, then why would Jesus let them walk away on a misunderstanding?
That ignores the meaning of substance or that the accidents of the bread and wine do not change. Were you able to travel back in time to Jerusalem in 33AD with all of the best forensic instruments and techniques known to man and were to encounter Jesus you would not be able to differentiate His human nature from His divine nature through any test using human senses. Next will you tell us that no one is born again because you do do not have a scientific test to prove it?