Greco-Roman Religion was politheistic, while Medieval Europe was monotheistic (Christian). While the Greek and Roman religion had many gods, Christian relgion only has one god.
Greco-Roman culture employed large amounts of slave labor, while in Medieval Europe, feudalism was the primary economic system, in which serfs, not slaves, were the primary source of labor.
Greco-Roman culture was mostly present in the Mediterranean world, while Medieval culture also had presence in Northern and Eastern Europe.
Two similarities:
Both Greco-Roman culture and medieval culture are European cultures. They took place in the European continent.
In Both cultures, the use of Latin (in Western Europe), and Greek (in Eastern Europe), continued to be important. Most scholars of medieval Europe wrote Greek or Latin.
