Ralph Waldo Emerson's famous essay "Self Reliance" is full of often-quoted phrases such as the following:
"Society everywhere is in conspiracy against the manhood of every one of its members. . . . The virtue in most request is conformity. Self-reliance is its aversion. It loves not realities and creators, but names and customs."
"Nothing can bring you peace but yourself."
In Henrik Ibsen's play An Enemy of the People are some good quotes, such as:
"You should never wear your best trousers when you go out to fight for freedom and truth."
"The strongest man in the world is he who stands alone."
The protagonist of Ibsen's play has to stand up against everyone in his community in defense of what he knows to be true.
One of the most famous quotes from Shakespeare is:
This above all: to thine own self be true, / And it must follow, as the night the day, / Thou canst not then be false to any man." (Hamlet, I,iii,75)