During the Middle Ages how did Western Christianty preserve elements of classical society and establish a foundation for cultural unity?In what...

This is a big question for a small space.  When the Roman Empire fell prey to barbarian invasion in the fifth century, the only institution strong enough to fill the void was the Roman Catholic church.  The church united present day Europe into an area referred to by its inhabitants as Christendom.  The church preserved not only the writings of the Romans, early Christian fathers and the Bible, but also, in its very structure of bishoprics and synods, the concepts of republican government and democracy.  When Edward III of England developed Parliament, he was modelling the synods of the church; he had no knowledge of the Roman Senate.

Unlike present day Christianity, the Roman Church was the only game in town for Medieval Christians.  In every town, in every village, masses were sung, sermons were preached, and lessons were taught; all in Latin.  There was no concept of nationhood during much of the medieval period.  Society was ordered not by nationality or state lines, but by a network of confusing relationships based on duties and obligations.  One did not live in France or England or Germany; one lived in Christendom.  The era was an Age of Faith in which faith in Christ was the unifying factor of society.  The church gave validity to the order of society by asserting that order was set by God.  Thus if you broke your obligation to your Lord, you also broke your obligation to God.  This went a long way to preserving stability in an unstable time.