“The battle must be fought exactly as the battle of religious toleration was fought. And as in that case, so in this, a decay in the intensity of belief is likely to prove the decisive factor. While men were convinced of the absolute truth of Catholicism or Protestantism, as the case may be, they were willing to persecute on account of them. While men are quite certain of their modern creeds, they will persecute on their behalf. Some element of doubt is essential to the practice, thought not to the theory, of toleration.”