I'm going to reproduce the entire letter from John Adams to Thomas Jefferson, January 23, 1825:
MY DEAR SIR,—We think ourselves possessed, or at least we boast that we are so, of liberty of conscience on all subjects and of the right of free inquiry and private judgment in all cases, and yet how far are we from these exalted privileges in fact. There exists, I believe, throughout the whole Christian world, a law which makes it blasphemy to deny, or to doubt the divine inspiration of all the books of the Old and New Testaments. from Genesis to Revelations. In most countries of Europe it is punished by fire at the stake, or the rack, or the wheel. In England itself, it is punished by boring through the tongue with a red-hot poker. In America it is not much better; even in our Massachusetts, which, I believe, upon the whole, is as temperate and moderate in religious zeal as most of the States, a law was made in the latter end of the last century, repealing the cruel punishments of the former laws, but substituting fine and imprisonment upon all those blasphemies upon any book of the Old Testament or New. Now, what free inquiry, when a writer must surely encounter the risk of fine or imprisonment for adducing any arguments for investigation into the divine authority of those books? Who would run the risk of translating Volney's Recherches Nouvelles? Who would run the risk of translating Dapin's? But I cannot enlarge upon this subject, though I have it much at heart. I think such laws a great embarrassment, great obstructions to the improvement of the human mind. Books that cannot bear examination, certainly ought not to be established as divine inspiration by penal laws. It is true, few persons appear desirous to put such laws in execution, and it is also true that some few persons are hardy enough to venture to depart from them; but as long as they continue in force as laws. the human mind must make an awkward and clumsy progress in its investigations. I wish they were repealed. The substance and essence of Christianity, as I understand it, is eternal and unchangeable, and will bear examination forever; but it has been mixed with extraneous ingredients, which, I think, will not bear examination, and they ought to be separated. Adieu.
Adams is quite clear that the unalienable right to liberty of conscience means the right to blaspheme or in particular to doubt the truth of the divine inspiration of the Bible, which Adams himself personally did. When Adams stated the Christian religion "has been mixed with extraneous ingredients, which, I think, will not bear examination, and they ought to be separated," an evangelical Protestant might hope he were referring only to Roman Catholicism. But this is wrong. Adams, himself a lifelong, committed theological unitarian believed the entire institution of orthodox Trinitarian Christianity was corrupted. And those "corruptions of Christianity" were defined by Adams' and Jefferson's spiritual mentor, Joseph Priestley, as the Trinity, Incarnation, Atonement, and plenary inspiration of scripture. The Bible itself was "corrupted" and Adams believed man had an unalienable right to use his reason to edit what he saw as "error" from the Bible exactly as Jefferson did. As Adams praised Jefferson for cutting up the Bible in this regard:
“I admire your Employment, in selecting the Philosophy and Divinity of Jesus and seperating it from all intermixtures. If I had Eyes and Nerves, I would go through both Testaments and mark all that I understand.”
John Adams to Thomas Jefferson, Nov. 14, 1813.
How this relates to today's dispute over "originalism." Most constitutional scholars agree that the original public meaning of constitutional words and principles trumps the "original intent" of the Framers. But sometimes those original principles, especially vaguely and broadly defined like "unalienable rights of conscience" are pregnant with implications (yes I know those words are not even in the Constitution, but rather in America's Founding era natural rights documents). When one examines the private intent and one sees key Founders repeatedly discussing what they hoped to achieve by founding America on particular principles (i.e., the rights of conscience means the right to profess openly heresy and blasphemy) and then this is exactly what happens (even though many folks in the populace during the time those principles were enunciated weren't quite on board with the plan), such is telling to say the least.