"Truths turn into dogmas the instant that they are disputed. Thus every man who utters a doubt defines a religion. And the scepticism of our time does not really destroy the beliefs, rather it creates them; gives them their limits and their plain and defiant shape. We who are Liberals once held Liberalism lightly as a truism. Now it has been disputed, and we hold it fiercely as a faith. We who believe in patriotism once thought patriotism to be reasonable, and thought little more about it. Now we know it to be unreasonable, and know it to be right. We who are Christians never knew the great philosophic common sense which inheres in that mystery until the anti-Christian writers pointed it out to us. The great march of mental destruction will go on. Everything will be denied. Everything will become a creed. It is a reasonable position to deny the stones in the street; it will be a religious dogma to assert them. It is a rational thesis that we are all in a dream; it will be a mystical sanity to say that we are all awake. Fires will be kindled to testify that two and two make four. Swords will be drawn to prove that leaves are green in summer. We shall be left defending, not only the incredible virtues and sanities of human life, but something more incredible still, this huge impossible universe which stares us in the face. We shall fight for visible prodigies as if they were invisible. We shall look on the impossible grass and the skies with a strange courage. We shall be of those who have seen and yet have believed."
Gilbert K. Chesterton, Heretics (London and New York: John Lane: The Bodley Head, 1905), 304-305 (XX. Concluding remarks on the importance of orthodoxy). Here it is in the reprint of 1909.