A lack of progress in the field of mental development in a society inevitably affects also its physical development. Christian medieval Europe was a place where free thinking was for a very long time a completely unneeded and unknown luxury. This fact did greatly hamper also its economical and social development.
The Christian historians have always wanted to explain away the complete lack of economical, social, scientific and even artistic progress in the deep medieval Europe between years 400 - 1200 with infectious diseases as the plaque and the political instability of the time.

They have of course have a valid point here. There really was infectious diseases and a lot of political instability. However, it is all too easy to forget that also the earlier Roman empire was ravaged by both infectious diseases and immense political upheavals in the form of a series bitter and savage civil wars between different aspirants for ruling the Roman empire.
The very same diseases and continued political instability were ravaging Europe also after Christianity lost its sole dominant mental position during the dawn of Age of Enlightenment in the 16th and 17th centuries. These factors did not prevent the progress in Western Europe anymore, when people did become less mentally dumbed down by religion.

In medieval Europe, the Catholic Church achieved a position where it had a total control over all information in the society, a situation quite like the one in modern Iran. As in modern Iran the greatest part of the population had no chance of ever knowing that things could have been differently. They did not know that there even was a different kind of knowledge in existence or ever had been.
However, there emerged a group of educated people who could ultimately get hold of ancient scientific findings of the Greeks, which the Catholic Church had vigorously tried so hide away from view.
These people could soon clearly see how many things had been in so many ways so much better before the iron rule of the dogmatic Only True Faith had been established. A deep admiration for ancient Greece and Rome was created in the process.

Wikipedia

It is a real paradox that the Crusades which were initiated by the Church itself were one of the prime causes for destroying the monopoly of knowledge that was for so long time being held by the church. The irony was that just the crusading knights far away in Palestine got to know about the achievements of the ancient Greeks that had been found and preserved by some of the Muslim scientists. This did happen in a time when these ideas were hidden secrets in the Christian world.

The homecoming crusaders then did often sow the seeds that would soon grow to a full bloom in the Renaissance. The main ingredient in whole of Renaissance was basically the deep admiration for the culture of the antique Greeks and Romans. However, the Catholic Church did not for a long time tolerate such things in the literature or poetry at all.
So at first the most important visible expressions of the Renaissance were seen only in the architecture and in the field of visual arts.

Renaissance did not in itself bring many visible or tangible changes to the ways the Western European societies were run and governed. However, it did lead into opening up of the overall spiritual and mental climate in Europe. Most importantly a few important thinkers got the first faint glimpses even of the very basic fact that there really existed also other ideas than the official truth of the Catholic Church.
Open challenge to the Catholic Church was still quite impossible at the time. A certain death simply awaited everyone who dared the doubt even any of the silliest of the doctrines that were propagated by the Catholic Church.

However, as a result of Renaissance there was a creation of a new and much freer spiritual atmosphere. It did at first create a new kind of blooming in visual arts, but not in literature or science. At the time, an extremely radical thing was the simple fact one could depict also other things than the scenes from the Bible or the lives of the saints, as was the case during the darkest hours of the Church’s iron rule.
It was a great achievement in itself that art could again also tell about other things than death, perennial sin and agonies waiting the non-believers.

Of course, the real opening up of the Medieval Europe was still in the future. However, Renaissance did pave the way for the Age Of Enlightenment. It did create the first extremely important cracks in the monolithic facade of the unified Catholic Church
From these cracks did develop the fissures that later did really break the all-encompassing earthly and most of all mental and psychological power of the Catholic Church. This development did in due time lead to a new kind blossoming of the Western European societies. This happened in a way which had not been seen after the demise of the free-spirited Greek and Roman societies of the Antiquity.

(This little essay was completely refurbished in 17th of April 2012)