Theodore Parker was a transcendentalist and Unitarian preacher, as well as one of the “Secret Six” who funded John Brown’s raid on Harpers Ferry. Parker's 1852 sermon “Justice and the Conscience” inspired Martin Luther King Jr.’s famous quote that “the arc of the moral universe is long but it bends toward justice.” Part of Parker’s sermon read,
"You see a continual and progressive triumph of the right. I do not pretend to understand the moral universe, the arc is a long one, my eye reaches but little ways. I cannot calculate the curve and complete the figure by the experience of sight; I can divine it by conscience. But from what I see I am sure it bends towards justice."
Parker was a radical abolitionist. He gave many rousing speeches in favor of abolition. He spoke at the Melodeon in Boston on the anniversary of the kidnapping and return to slavery of Thomas Sims. After Anthony Burns was also returned to slavery in 1854, Parker gave a sermon at the Boston Music Hall in which he called the Fugitive Slave Act a “new crime against humanity.”