It is an old Quaker teaching that urges followers to stand up and speak to governments, rulers and the powerful about wrongs, abuses, injustice and unpleasant truths. Anita Hill used it for the title of her book.
Didn't know it was Quaker in origin. This slogan was a cliche during the civil rights movement of the 1960s. It meant that the speaker was convinced he was right (truth) and was about to prove it by seizing some sort of power.