How easy it is to make people believe a lie, and how hard it is to undo that work again!

Mark Twain (1835-1910) American writer [pseud. of Samuel Clemens]

Dictation (1906-12-02), The Autobiography of Mark Twain, Vol. 2 (2013)
    (Source)

A phrase that may be the origin the spurious Twain quotation, "It's easier to fool people than to convince them that they have been fooled."