“Each progressive spirit is opposed by a thousand mediocre minds appointed to guard the past.”

is this not supporting 1984 lets call it the minster of history if you like

I believe the past holds much truth that should be respected and guarded.  It is the 'Traditions and accepted procedures' of the past that the midling minds of the present are so quick to mindlessly guard.  It is the duty of historians and philosophers to examine our history and develop a guide map toward the future that deals honestly with the needs and actions of the past, the present, and the future.   

I believe that M.Maeterlinck likely meant this.  It isoftne hard to take a quote out of context and transfer its meaning as the author intended.   Particularly if it moves through translation.   

I don't know how to rate this. Not enough context, as a stand a lone communication to measure it to liberty.

I agree with Mike.  Is the author referring to ideas which are really progressive or ideas the proponent wants us to believe are progressive?  The people who today want us to call them progressive and liberal are still selling the ideas of socialism which have been proven to make false promises every time they are tried.  We even celebrate Thanksgiving which started as a celebration of survival under freedom of enterprise after abandoning the false promises of common wealth (as in the Commonwealth of Pennsylvania).

'Progressive' in the literal sense will indeed be opposed by those attached to the ways of the past.  The word 'progress' has been hijacked by socialists to mean the furtherance of socialist ideals and policies that of course are opposed by those that hold tightly to their rights and property while 'progressives' attempt to lay claim to them as common wealth. 

Just as science is not settled, nor is our understanding of the world; real progress is furthering the principles of liberty and the responsibilities that come with it.  Servitude to the State-Church is traditional and customary — and also the source of revolution, ending the reign of monarchs and dictators, empowering the People to be the noble sovereigns they are.  Now that's progress, and yes, there are "a thousand mediocre minds appointed to guard" their own servitude.  Mediocre minds are easily ruled and drown out truth.