Jesus is the perfect sacrifice prefigured in the Passover. There were four cups in the ritual. The third was the Todah (thanksgiving or Eucharista in Greek) offering of bread and wine. Jesus gave us His Body and His Blood as the passover lamb. In Jewish tradition, in the days of the Messiah, only the Todah offering would continue... and it does at every celebration of the Eucharist.
The fourth cup is the cup of consummation. Jesus, acting as priest and sacrifice on the cross completed the passover sacrifice with the drinking from the sponge. He then said, "It is finished." and gave up His Spirit in death.
John 19:30 Therefore when Jesus had received the sour wine, He said, " It is finished!" And He bowed His head and gave up His Spirit.
It is no more complicated than what is written in the text of Scripture. Your theologian went through some amazing theological gymnastics to avoid the 2,000 year old Catholic understanding... because obviously anything the Catholics believe must be wrong...
I'm sorry to argue with you. If I'd have known you were full of years of Catholic doctrine instead of an open mind to facts I would not have got into this discussion with you at all. Every one knows you can't argue with the brain washed. 2000 years of belief in something wrong does not make it a reality. Transubstantiation for instance.