This quotation comes from Edmund Burke's speech on The French Revolution. Burke was an Irishman, educated in London, but he took a keen interest in what was going on in France. Unlike many of his contemporaries, notably Thomas Paine, Burke was opposed to the French Revolution. He felt that it was destructive to the absolute core principles of French society, and more broadly, European society. This is what he is trying to emphasize in the quotation you have given.
Effectively, Burke is saying that the manners and the strictures of European civilization have always been held together by what he calls "the spirit of a gentleman and the spirit of religion." These two elements are represented by the nobility and the clergy. By tearing down the church and the aristocracy then, as the French Revolution had done, the revolutionaries were attacking the very basis of their civilization.
Burke goes on to state that it is through the existence of the nobility and the clergy in Europe that learning has been able to prevail, advancing societies and maintaining ideals. Between them, the nobility and the clergy, according to Burke, developed and protected learning, "enlarging" the ideas of wider society. Burke suggests that those who have benefited from this learning have become unable to recognize where it came from in the first place—that is, they are, essentially, ungrateful to these two great bodies in society for having helped them to improve themselves, and now no longer "know their place," but want to overthrow these two pillars of society. The revolutionaries, Burke is saying, do not realize that if they dismantle the pillars of their society, the society itself will surely collapse.