Tom Paine..................................

     A historical study of the eighteenth century Europe and America, without the study of Tom Paine, would be no study at all.  While not casting a judgement on the old saw "the pen is mightier than the sword,"  without Paine's writings many swords would not have been drawn.  Not sure that I would call him a radical, but certainly his thinking lay outside of the mainstream of western thought of his (or any other?) day, and he was a fearless writer of his thoughts.  Paine's most famous work, Common Sense, inflamed the Colonies towards the path of independence.  Later works, The Rights of Man, written in defense of the French Revolution, and The Age of Reason, a written assault on the organized church, and Agrarian Justice, conveying Paine's ideas about property, were no less inflammatory - just with different targets.  His works deserve to be read in any age; just prepare to be inflamed.  (It strikes me as amazing that all four of these pamphlets/books can be acquired via a few keystrokes and the exchange of only a few dollars via Kindle.)
     Here are a few of Paine's written thoughts:

The supposed quietude of a good man allures the ruffian; while on the other hand, arms, like law, discourage and keep the invader and the plunderer in awe, and preserve order in the world as well as property

Where knowledge is a duty, ignorance is a crime.

Some writers have so confounded society with government, as to leave little or no distinction between them; whereas they are not only different, but have different origins. Society is produced by our wants, and government by our wickedness; the former promotes our happiness POSITIVELY by uniting our affections, the latter NEGATIVELY by restraining our vices. The one encourages intercourse, the other creates distinctions. The first a patron, the last a punisher.

Society in every state is a blessing, but government even in its best state is but a necessary evil; in its worst state an intolerable one;  for when we suffer, or are exposed to the same miseries BY A GOVERNMENT, which we might expect in a country WITHOUT GOVERNMENT, our calamity is heightened by reflecting that we furnish the means by which we suffer.

Of more worth is one honest man to society and in the sight of God, than all the crowned ruffians that ever lived.

When we are planning for posterity, we ought to remember that virtue is not hereditary.

A long habit of not thinking a thing wrong, gives it a superficial appearance of being right, and raises at first a formidable outcry in defense of custom.

All national institutions of churches, whether Jewish, Christian, or Turkish, appear to me no other than human inventions set up to terrify and enslave mankind, and monopolize power and profit.

If there must be trouble, let it be in my day, that my child may have peace.

An avidity to punish is always dangerous to liberty. It leads men to stretch, to misinterpret, and to misapply even the best of laws. He that would make his own liberty secure must guard even his enemy from oppression; for if he violates this duty he establishes a precedent that will reach to himself.

Men did not make the earth... It is the value of the improvements only, and not the earth itself, that is individual property... Every proprietor owes to the community a ground rent for the land which he holds.

for all the principles of science are of divine origin. Man cannot make, or invent, or contrive principles; he can only discover them, and he ought to look through the discovery to the Author.