One that secularists like to trot out. “Christianity is the most perverted system that ever shone on man.” Monticello notes this quotation as "so far not been found in any of the sources available to us." However, they, apparently, haven't found that actual primary source from which this quotation was paraphrased. They note Thomas Jefferson to William Baldwin, 19 January 1810, and Thomas Jefferson to Ezra Stiles Ely, Monticello, 25 June 1819. The actual source, without question, is Thomas Jefferson to Joseph Priestley, 21 March 1801. The relevant part reads:
Those who live by mystery and charlatanerie, fearing you would render them useless by simplifying the Christian philosophy,—the most sublime and benevolent, but most perverted, system that ever shone on man,—endeavored to crush your well-earned and well-deserved fame.
This illustrates the nuanced context that most folks miss; the notion is Christianity had been “perverted” by its orthodox doctrines like the Trinity, Incarnation and Atonement and once stripped away contains some of the most benevolent sublime moral teachings. This is clearly what Jefferson, J. Adams, and Franklin believed and probably what Washington, Madison and others believed.