… is from page 580 of the 1988 collection of Lord Acton’s writings and notes to himself (edited by the late J. Rufus Fears), Essays in Religion, Politics, and Morality; specifically, it’s a note drawn from Acton’s extensive papers at Cambridge University:
Every doctrine to become popular, must be made superficial, exaggerated, untrue. We must always distinguish the real essence from the conveyance, especially in political economy.
