“I almost shudder at the thought of alluding to the most fatal example of the abuses of grief which the history of mankind has preserved - the Cross. Consider what calamities that engine of grief has produced! With the rational respect that is due to it, knavish priests have added prostitutions of it, that fill or might fill the blackest and bloodiest pages of human history.{Letter to Thomas Jefferson, September 3, 1816]”

“This society [Jesuits] has been a greater calamity to mankind than the French Revolution, or Napoleon's despotism or ideology. It has obstructed the progress of reformation and the improvement of the human mind in society much longer and more fatally.{Letter to Thomas Jefferson, November 4, 1816. Adams wrote an anonymous 4 volume work on the destructive history of the Jesuits}”

“But how has it happened that millions of fables, tales, legends, have been blended with both Jewish and Christian revelation that have made them the most bloody religion that ever existed? How has it happened that all the fine arts, architecture, painting, sculpture, statuary, music, poetry, and oratory, have been prostituted, from the creation of the world, to the sordid and detestable purposes of superstition and fraud?[Letter to judge F.A. Van der Kamp, December 27, 1816.]”

“Can a free government possibly exist with the Roman Catholic religion?{Letter to Thomas Jefferson, May 19, 1821}”

“I am bold to Say that neither you nor I, will live to See the Course which 'the Wonders of the Times' will take. Many Years, and perhaps Centuries must pass, before the current will acquire a Settled direction... yet Platonic, Pythagoric, Hindoo, and cabalistic Christianity, which is Catholic Christianity, and which has prevailed for 1,500 years, has received a mortal wound, of which the monster must finally die. Yet so strong is his constitution, that he may endure for centuries before he expires.{Letter to Thomas Jefferson, July 16 1814}”

“...Turn our thoughts, in the next place, to the characters of learned men. The priesthood have, in all ancient nations, nearly monopolized learning. Read over again all the accounts we have of Hindoos, Chaldeans, Persians, Greeks, Romans, Celts, Teutons, we shall find that priests had all the knowledge, and really governed all mankind. Examine Mahometanism, trace Christianity from its first promulgation; knowledge has been almost exclusively confined to the clergy. And, even since the Reformation, when or where has existed a Protestant or dissenting sect who would tolerate a free inquiry? The blackest billingsgate, the most ungentlemanly insolence, the most yahooish brutality is patiently endured, countenanced, propagated, and applauded. But touch a solemn truth in collision with a dogma of a sect, though capable of the clearest proof, and you will soon find you have disturbed a nest, and the hornets will swarm about your legs and hands, and fly into your face and eyes.[Letters to John Taylor, 1814, XVIII, p. 484]”

“Government has no right to hurt a hair on the head of an Atheist for his Opinions. Let him have a care of his Practices.{Letter to his son and future president, John Quincy Adams, 16 June 1816}”