It is dangerous to be sincere unless you are also stupid.

George Bernard Shaw (1856 - 1950), Man and Superman (1903) "Maxims for Revolutionists"
The secret of success is sincerity. Once you can fake that you've got it made.

Jean Giraudoux (1882 - 1944)
Judge thyself with the judgment of sincerity, and thou will judge others with the judgment of charity.

John Mitchell Mason
I am not sincere, even when I say I am not.

Jules Renard (1864 - 1910)
Sincerity is the way of Heaven.

Mencius (371 BC - 289 BC), Works
A little sincerity is a dangerous thing, and a great deal of it is absolutely fatal.

Oscar Wilde (1854 - 1900), The Critic as Artist, part 2, 1891