This is a very interesting letter. In the beginning Adams doubts we have the right version of the Ten Commandments. He states the Bible contains "error or amendment." Then he praises Jefferson's Bible where Jefferson cut out what he saw as error. Here is the larger context of the quote. I have emphasized Adams claiming his desire to make his own "Adams' Bible."
I admire your employment in selecting the philosophy and divinity of Jesus, and separating it from all mixtures. If I had eyes and nerves I would go through both Testaments and mark all that I understand. To examine the Mishna, Gemara, Cabbala, Jezirah, Sohar, Cosri and Talmud of the Hebrews would require the life of Methuselah, and after all his 969 years would be wasted to very little purpose. The daemon of hierarchical despotism has been at work both with the Mishna and Gemara. In 1238 a French Jew made a discovery to the Pope (Gregory 9th) of the heresies of the Talmud. The Pope sent thirty-five articles of error to the Archbishops of France, requiring them to seize the hooks of the Jews and burn all that contained any errors. He wrote in the same terms to the kings of France, England, Arragon, Castile, Leon, Navarre and Portugal. In consequence of this order, twenty cartloads of Hebrew books were burnt in France ; and how many times twenty cartloads were destroyed in the other kingdoms ? The Talmud of Babylon and that of Jerusalem were composed from 120 to 500 years after the destruction of Jerusalem.
If Lightfoot derived light from what escaped from Gregory's fury, in explaining many passages in the New Testament, by comparing the expressions of the Mishna with those of the Apostles and Evangelists, how many proofs of the corruptions of Christianity might we find in the passages burnt?
I do think, after reading Adams in great detail, his partially inspired Bible would be much thicker than Jefferson's. However, he still believed that the biblical canon contained more than a nominal amount of "error or amendment" that man's reason should edit.