When giving his reasons for why he believed the Bible was an errant, corrupted text, Adams goes off on a conspiracy tangent. As he wrote to Jefferson July 9, 1813:
No sooner has one party discovered or invented any amelioration of the condition of man, or the order of society than the opposite party belies it, misconstrues it, misrepresents it, ridicules it, insults it, and persecutes it. Records are destroyed. Histories are annihilated or interpolated or prohibited; sometimes by Popes, sometimes by Emperors, sometimes by aristocratical, and sometimes by democratical assemblies, and sometimes by mobs.
Aristotle wrote the history and description of eighteen hundred republics which existed before his time. Cicero wrote two volumes of discourses on government, which, perhaps, were worth all the rest of his works. The works of Livy and Tacitus, &c., that are lost, would be more interesting than all that remain. Fifty gospels have been destroyed. Where are St. Luke’s world of books that were written?
If you ask my opinion, who has committed all the havoc? I will answer you candidly. Ecclesiastical and imperial despotisms have done it to conceal their frauds.
Why are the histories of all nations, more ancient than the Christian era, lost? Who destroyed the Alexandrian library? I believe that Christian priests, Jewish rabbis, Grecian sages, and Roman emperors, had as great a hand in it as Turks and Mahometans. Democrats, rebels, and Jacobins, when they possess a momentary power, have shown a disposition both to destroy and to forge records, as Vandatical as priests and despots. Such has been and such is the world we live in.
And the following is from an interesting letter to F.A. Vanderkemp Dec. 27, 1816:
Jesus is benevolence personified, an example for all men. Dupuis has made no alteration in my opinions of the Christian religion, in its primitive purity and simplicity, which I have entertained for more than sixty years. It is the religion of reason, equity, and love; it is the religion of the head and of the heart.
Christianity, you will say, was a fresh revelation. I will not deny this. As I understand the Christian religion, it was, and is, a revelation. 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?
The eighteenth century had the honor to discover that Ocellus of Lucania, Timæus of Locris, Aristotle, Tacitus, Quintilian, and Pliny, were in the right. The philosophy of Frederic, Catharine, Buffon, De la Lande, Diderot, d’Alembert, Condorcet, d’Holbach, and Dupuis, appears to me to be no more nor less than the philosophy of those ancient men of science and letters, whose speculations came principally from India, Egypt, Chaldea, and Phœnicia. A consolatory discovery, to be sure! Let it once be revealed or demonstrated that there is no future state, and my advice to every man, woman, and child would be, as our existence would be in our own power, to take opium. For, I am certain, there is nothing in this world worth living for but hope, and every hope will fail us, if the last hope, that of a future state, is extinguished.
You should read the entirety of both letters for more context. I excerpted what I thought was important to get Adams' view of "Christianity."