Joseph Priestly, The Corruption of Christianity
An excellent biography of Joseph Priestly is at this link.
His most famous work is the Corruptions of Christianity in 1782. Wikipedia's article on the book itself is at this link. It appears typically in 2 volumes. The volume 1 in text appears at archives.org at this link. You can also read it at this link.
Of this work of 1782, Thomas Jefferson wrote (see Digital Collection of the Library of Congress):
I have written to Philadelphia for Doctor Priestley's History of the Corruptions of Christianity, which I will send to you, and recommend to an attentive perusal, because it establishes the groundwork of my view of this subject. (TJ to Martha Jefferson, 25 April 1803)
The work of Dr. Priestley which I sent you has always been a favorite of mine. I consider the doctrines of Jesus as delivered by himself to contain the outlines of the sublimest system of morality that has ever been taught but I hold in the most profound detestation and execration the corruptions of it which have been invented by priestcraft and established by kingcraft constituting a conspiracy of church and state against the civil and religious liberties of mankind. (TJ to Henry Fry, 17 June 1803)
I have read his Corruptions of Christianity and Early Opinions of Jesus over and over again; and I rest on them...as the basis of my own faith. These writings have never been answered, nor can be answered, by quoting historical proofs, as they have done. For these facts therefore I cling to their learning, so much superior to my own. (TJ to John Adams, 22 August 1813)(books.google link)
Priestly once wrote a letter to Jefferson, which original is digitized at the Library of Congress. It is hard to read.
President Washington wrote Priestly a complimentary letter, it appears. It too is digitized at the Library of Congress.
Jefferson wrote Priestly another time -- reflecting his agreement with the many corruptions of Scripture that Priestly exposed. Jefferson wrote Priestly:
"...those who live by mystery & charlatanerie, fearing you would render them useless by simplifying the Christian philosophy, the most sublime & benevolent, but most perverted system that ever shone on man, endeavored to crush your well earnt, & well deserved fame." - Thomas Jefferson to Joseph Priestley, Washington, March 21, 1801 (PTJ, 33:393. Press copy available from the Library of Congress. Ford transcription available online. Source: Jefferson of Monticello.
Corruptions of Christianity
Let's explore what Priestly wrote to find out what Jefferson so admired in it. I have the printed edition of London: The British and Foreign Unitarian Association, 1871. This is a reprint of the 1782 edition.
In the Preface of 1871, it says Priestly "was a firm believer in the supernatural power and divine mission of Chirst." (Page 1.)
In the Preface to the Rutt's edition of 1818, Rutt says Priestly was "not one of the credulous Protestants who satisfy themselves that the Reformers in the sixteenth century had left no corruption of Christianity unreformed."
Dr. Priestly's Preface says Christianity as known by Jews, Muslims, and others is a "corrupted" kind, and attached to a "worldly interest" that there is no surprise "that mankind in general...can be prevailed upon to consider the evidence that is alleged in its favor." Id., at page x. Priestly believes by showing the corruptions of Christianity to non-believers that this will remove one of the "primary causes of infidelity." Id., at xi. Priestly insists that he is "deeply concerned for the religion I profess." Id., at xii.
Priestly believes the evidence of the corruptions of Christianity for "what so long has passed for Christianity, the more highly will [Christians] esteem what is truly so....
Priestly defines a corruption as anything which is a "departure from the original scheme, or an innovation." Id., at page xi.
Part I, The History of Opinions Relating to Jesus Christ
Priestly says that the most important truth protected by Judaism was the unity of God. And the expectation of Messiah was solely one "descended from the tribe of Judah, and the family of David, a person in whom themselves and all nations of the earth should be blessed," and then insists:
"but none of their prophets gave them an idea of any other than a man like themselves in that illustrious charachter...." Id., at 1.
The apostles regarded Jesus as a "man approved by God, by wonders and signs which God did by him." Acts 2:22. "Peter uses the simple language above quoted of a man approved by God, immediately after the descent of the Holy Spirit." Id. And Paul in 1 Tim. 2:5 says "There is one God, and one mediator, between God and men, the man Jesus Christ." Priestly continues: "He does not say the God, or the God-man, or the super-angelic being, but simply the man Christ Jesus." Id, at 1-2.
Priestly digresses and says in the earliest church was a group of Jewish Christians called Ebionites. They were despised by the Jewish establishment. It may be surmised from Origen, Eusebius and Epiphanius that they were also called the Nazarenes. Id. at 3.
Some Ebionites believed Jesus was the son of Mary and Joseph, while other believed he had a miraculous birth. Priestly cites Epiphanius, Opera 1682 Vol. I at 123, 125. Epiphanius' account of the Ebionites "makes no mention of any of them believing the divinity of Christ in any sense of the word." Id. at page 3. Here, Priestly is not careful in the use of terms. They did not speak of any deity of Christ, but they did speak of a divine indwelling of Jesus at the baptism of John.
A most telling observation is made by Priestly about the Ebionites likely being orthodox and nonheretical for centuries up through Epiphanius's era of late 300s. Priestly notes the earlier words of Hegesippus (a.d. 170) which talk of numerous heretics, but nothing is said about the Ebionites being heretics. Id, at page 3. Instead, Hegesippus "makes no mention of the supposed heresy of the Nazarenes or Ebionites but says that, in his travels to Rome, where he spent some time with Anicetus, and visited the bishops of other sees, he found that they all held the same doctrine that was taught in the Law, by the prophets, and by our Lord." Id., at page 3, citing Euseb. Hist. 1720 L iv. C xxii page 181-182.
Priestly notes that Valesius, the "translator of Eusebius," said the works of Hegesippus were not preserved because of "errors" in them, which Priestly says was no doubt the belief in the oneness of God.
Tertullian in the 200s says the Docetists say Jesus only "appeared to be a man" while the Ebionites believed Jesus "was no more than a man." Id. at 4.
Austin (Augustine) believed the Docetists (Marcionites) believed Jesus was God, but the "Ebionites "believed him to be a man, but denied he was God." Id., at 4.
Austin / Augustine interestingly admitted he himself had the same opinion of the Ebionites "till he became acquainted with the writings of Plato, which in his time were translated into Latin and in which he learned the doctrine of the Logos." Id., at 4. Thus, Augustine started his Christian path with an Ebionite view, but influenced by Plato he changed to a view of Jesus as the Logos of Plato.