My favourite medieval authors are:

Robert Grossesteste because he studied the Bible in the original languages, believed the job of the priest was to preach and teach, and gave trenchant criticisms of the papacy.

Thomas Bradwardine because he powerfully understood the grace of God.

John Wyclif because his theology anticipated many reformation themes.

Positives : the developments within the doctrine of God particularly Duns Scotus, who presented a more personal understanding of reality (which paved the way for Luther).

Negatives :

[1] The new claims made by the papacy under the influence of Gregory VII (seeing a high point in Innocent III).

[2] Salvation was obscured by the rise of the doctrine of the works of satisfaction.

Luther and Calvin believed if there was a point when the Western Church fell, it wasn't with Constantine it was with the rise of the papacy in the 11th century under Hildebrand's (Gregory VII) influence. This was (to them) the rise of Anti-Christ.

Blessings.

__________________
Marty

"There is nothing so necessary to draw us to repentance as good thoughts of God." (Thomas Manton)