“Once you know the truth, it's always obvious”

Also attributed to Robert H. Schuller

1790s, First Principles of Government (1795)
Context: It is never to be expected in a revolution that every man is to change his opinion at the same moment. There never yet was any truth or any principle so irresistibly obvious that all men believed it at once. Time and reason must cooperate with each other to the final establishment of any principle; and therefore those who may happen to be first convinced have not a right to persecute others, on whom conviction operates more slowly. The moral principle of revolutions is to instruct, not to destroy.

Knowing Yourself: The True in the False (1996)

The Deliverance from Error https://www.amazon.com/Al-Ghazalis-Path-Sufism-Deliverance-al-Munqidh/dp/1887752307

As quoted in Commentary: The gaffer speaks, The Times, April 23, 1988.

as quoted by K.C. Cole, Sympathetic Vibrations: Reflections on Physics as a Way of Life (1985)

The Rosicrucian Cosmo-Conception

The character "Luka" in The Lower Depths (1902) English translation by Laurence Irving (1912)
Context: There — you say — truth! Truth doesn't always heal a wounded soul. For instance, I knew of a man who believed in a land of righteousness. He said: "Somewhere on this earth there must be a righteous land — and wonderful people live there — good people! They respect each other, help each other, and everything is peaceful and good!" And so that man — who was always searching for this land of righteousness — he was poor and lived miserably — and when things got to be so bad with him that it seemed there was nothing else for him to do except lie down and die — even then he never lost heart — but he'd just smile and say: "Never mind! I can stand it! A little while longer — and I'll have done with this life — and I'll go in search of the righteous land!" — it was his one happiness — the thought of that land. And then to this place — in Siberia, by the way — there came a convict — a learned man with books and maps — yes, a learned man who knew all sorts of things — and the other man said to him: "Do me a favor — show me where is the land of righteousness and how I can get there." At once the learned man opened his books, spread out his maps, and looked and looked and he said — no — he couldn't find this land anywhere... everything was correct — all the lands on earth were marked — but not this land of righteousness. The man wouldn't believe it.... "It must exist," he said, "look carefully. Otherwise," he says, "your books and maps are of no use if there's no land of righteousness." The learned man was offended. "My plans," he said, "are correct. But there exists no land of righteousness anywhere." Well, then the other man got angry. He'd lived and lived and suffered and suffered, and had believed all the time in the existence of this land — and now, according to the plans, it didn't exist at all. He felt robbed! And he said to the learned man: "Ah — you scum of the earth! You're not a learned man at all — but just a damned cheat!" — and he gave him a good wallop in the eye — then another one... [After a moment's silence. ] And then he went home and hanged himself.