Indeed the Middle Ages were at least 600 years long, 800 or so according to some historians, and the Christian population varied hugely during that period. Note Muslim Spain for much of that period. And it depends on where you draw the borders of Europe - especially the eastern border and to some extent the northern border in Scandinavia (Vikings etc - Vikings became Christianised slowly).
Also it depends on whether you count all forms of Christianity: Irish Christianity, Roman Catholic, Orthodox, etc. Protestantism started to come along at the very end of the Middle Ages.
It also depends on whether you count "nominal" Christians who very seldom turn up at churches, or who do not really believe, but find it socially useful to claim that they are Christians.
As with all large-scale questions covering large geographical areas and time frames - there is no simple answer! If your teacher says that there is a simple answer then he/she is simply "teaching to the book", not taking account of reality.