“A nation can survive its fools and even the ambitious. But it cannot survive treason. An enemy at the gates is less formidable, for he is known and carries his banner openly. But the traitor moves amongst those within the gate freely, his sly whispers rustling through the alleys, heard in the very halls of government itself. For the traitor appears not a traitor; he speaks in accents familiar to his victims; and he wears their face and their arguments. He appeals to the baseness that lies deep within the hearts of all men. He rots the soul of the nation secretly and unknown in the night to undermine the pillars of the city. He infects the body politic, so that it can no longer resist. A murderer is less to fear.” – Marcus Tullius Cicero, 42 B.C.

SOURCE: http://quotes.liberty-tree.ca/quote_blog/Marcus.Tullius.Cicero.Quote.B6EA
Marcus Tullius Cicero (106-43 B.C.) Roman Statesman, Philosopher and Orator
Attributed to a speech in the Roman Senate in 58 BC as “Recorded by Sallust” in the fictional novel ‘A Pillar of Iron,’ by Taylor Caldwell (1983), ch. 5. The quotation bears resemblance to Cicero’s Second Oration against Cataline. [2.11]
Image source: http://cdn.history.com/sites/2/2014/01/roman-forum-1.jpg