Having now read Justo Gonzalez' The Story of Christianity, I am condemned to remember the following every time I hear the phrase, "Be reasonable!" This event seems to have been sanitized from everything I previously learned about the French Revolution.
The French Revolution created its own religion, called first the Cult of Reason, and later the Cult of the Supreme Being. ... The revolution wished to have nothing to do with the church. Even the calendar was changed, giving way to a more “reasonable” one in which weeks had ten days and months were named after conditions of nature in each season—“Thermidor,” “Germinal,” “Fructidor,” and so forth. Great ceremonies were also developed to take the place of religious festivals—beginning with the solemn procession taking Voltaire’s remains to the Pantheon of the Republic. Then temples to Reason were built, and an official list of saints was issued—which included Jesus, Socrates, Marcus Aurelius, and Rousseau. Other rites were prescribed for weddings, the dedication of children to Freedom, and funerals. All this would have been merely ridiculous, were it not for its cost in suffering and bloodshed. The promoters of the new religion made use of the guillotine with cruel liberality.
Religions that grow over time, out of human need and experience, have all been afflicted by great cruelty. However, I also see in them sincere efforts by people to reach out to something transcendent and superior to themselves, and often evidence of God's efforts to reveal His own nature. But attempts by human beings to create new religions all at once out of whole cloth always seem disassociated from reality, hence ridiculous, and man-centered, hence selfish.