OPPS! I'm not sure I have it right, but it is close. It is truth.
(1) "The condition upon which God hath given liberty to man is eternal vigilance; which condition if he break, servitude is at once the consequence of his crime and the punishment of his guilt." John Philpot Curran, 1790
(2) "Eternal vigilance is the price of liberty." Wendell Phillips, 1852
My own way of saying it is simply, "The price of liberty is eternal vigilance."