As was noted in “Jesus in Theosophical History” in Theosophical History, Vol 1, No 3, July, 1985:38-45, the 100 B.C. theory (the precise date is sometimes given differently) was introduced by H.P. Blavatsky in Isis Unveiled (1877):ISIS
The Jewish version of the birth of Jesus is recorded in the Sepher-Toldos Jeshu in the following words:
“Mary having become the mother of a Son, named Jehosuah, and the boy growing up, she entrusted him to the care of the Rabbi Elhanan, and the child progressed in knowledge, for he was well gifted with spirit and understanding.
“Rabbi Jehosuah, son of Perachiah, continued the education of Jehosuah (Jesus) after Elhanan, and initiated him in the secret knowledge”; but the King, Janneus, having given orders to slay all the initiates, Jehosuah Ben Perachiah, fled to Alexandria, in Egypt, taking the boy with him.
While in Alexandria, continues the story, they were received in the house of a rich and learned lady (personified Egypt). Young Jesus found her beautiful, notwithstanding “a defect in her eyes,” and declared so to his master. Upon hearing this, the latter became so angry that his pupil should find in the land of bondage anything good, that “he cursed him and drove the young man from his presence.” Then follow a series of adventures told in allegorical language, which show that Jesus supplemented his initiation in the Jewish Kabala with an additional acquisition of the secret wisdom of Egypt. When the persecution ceased, they both returned to Judea.[Footnote: Eliphas Levi ascribes this narrative to the Talmudist authors of “Sota” and “Sanhedrin,” p. 19, book of “Jechiel.”]
The real grievances against Jesus are stated by the learned author of Tela Ignea Satanae (the fiery darts of Satan) to be two in number: 1st, that he had discovered the great Mysteries of their Temple, by having been initiated in Egypt; and 2d, that he had profaned them by exposing them to the vulgar, who misunderstood and disfigured them. This is what they say:[Footnote: This fragment is translated from the original Hebrew by Eliphas Levi in his “La Science des Esprits.”]
“There exists, in the sanctuary of the living God, a cubical stone, on which are sculptured the holy characters, the combination of which gives the explanation of the attributes and powers of the incommunicable name. This explanation is the secret key of all the occult sciences and forces in nature. It is what the Hebrews call the Scham hamphorash. This stone is watched by two lions of gold, who roar as soon as it is approached.[Footnote: Those who know anything of the rites of the Hebrews must recognize in these lions the gigantic figures of the Cherubim, whose symbolical monstrosity was well calculated to frighten and put to flight the profane.] The gates of the temple were never lost sight of, and the door of the sanctuary opened but once a year, to admit the High Priest alone. But Jesus, who had learned in Egypt the ‘great secrets’ at the initiation, forged for himself invisible keys, and thus was enabled to penetrate into the sanctuary unseen. . . . He copied the characters on the cubical stone, and hid them in his thigh; after which, emerging from the temple, he went abroad and began astounding people with his miracles. The dead were raised at his command, the leprous and the obsessed were healed. He forced the stones which lay buried for ages at the bottom of the sea to rise to the surface until they formed a mountain, from the top of which he preached.” The Sepher Toldos states further that, unable to displace the cubical stone of the sanctuary, Jesus fabricated one of clay, which he showed to the nations and passed it off for the true cubical stone of Israel.
This allegory, like the rest of them in such books, is written “inside and outside” — it has its secret meaning, and ought to be read two ways. The kabalistic books explain its mystical meaning. Further, the same Talmudist says, in substance, the following: Jesus was thrown in prison, and kept there forty days; then flogged as a seditious rebel; then stoned as a blasphemer in a place called Lud, and finally allowed to expire upon a cross. “All this,” explains Levi, “because he revealed to the people the truths which they (the Pharisees) wished to bury for their own use. He had divined the occult theology of Israel, had compared it with the wisdom of Egypt, and found thereby the reason for a universal religious synthesis.”
However cautious one ought to be in accepting anything about Jesus from Jewish sources, it must be confessed that in some things they seem to be more correct in their statements (whenever their direct interest in stating facts is not concerned) than our good but too jealous Fathers. One thing is certain, James, the “Brother of the Lord,” is silent about the resurrection. He terms Jesus nowhere “Son of God,” nor even Christ-God. Once only, speaking of Jesus, he calls him the “Lord of Glory,” but so do the Nazarenes when writing about their prophet Iohanan bar Zacharia, or John, son of Zacharias (St. John Baptist). Their favorite expressions about their prophet are the same as those used by James when speaking of Jesus. A man “of the seed of a man,” “Messenger of Life,” of light, “my Lord Apostle,” “King sprung of Light,” and so on. “Have not the faith of our Lord JESUS Christ, the Lord of Glory,” etc.,
H.P. Blavatsky Isis Unveiled: A Master-Key to the Mysteries of Ancient and Modern Science and Theology (1877) Vol. 2 p. 201-202.The text of Isis Unveiled is available on-line at: http://www.theosociety.org/pasadena/isis/iu-hp.htm
Blavatsky quotes Eliphas Levi La Science Des Esprits. Revelation du Dogme Secret des Kabbalistes Espirit Occulte des Evangiles Apfregution des Doctrines et des Phenomenes Spirites Germer Balliere, Paris, 1865.
A digital version of the book is available on-line at: https://archive.org/details/lasciencedesesp00lvgoog
His book has not been translated, but a copy is in the Society for Psychical Research Library in London.
Éliphas Lévi (Alphonse Louis Constant)(1810-1875) was a French writer on occultism and ceremonial magic. Having trained for the Roman Catholic Priesthood, he left the seminary after being ordained a Deacon, having both fallen in love and become fascinated with esotericism. For Levi see:
Christopher McIntosh, Christopher Eliphas Lévi and the French Occult Revival Rider, London, 1975
Thomas A Williams Eliphas Levi, Master of the Cabala, the Tarot and the Secret Doctrines Venture Press, Savannah GA, 2003
Levi’s argument for the 100BC theory is based on the Sefer Toledot Yeshum so presumably he had access to a text of that work, or to translations of it. There are at least a hundred manuscripts of the Sefer Toledot Yeshu, almost all of them late medieval, the oldest manuscript being from the 11th century. The text was first translated into Latin by Ramón Martí (or Martini), a Dominican Friar, toward the end of the 13th century.
Versions to which Levi might have access were that published by Johann Christian Wagenseil (1633-1705) in 1681, a Hebrew text of the Toledot Yeshu with a Latin translation, in a book titled Tela Ignea Satanae (“Satan’s Flaming Arrow”), to which Blavatsky refers in the quotation cited above, or that published by Johann Jacob Huldreich (or Huldrich) in Leyden, Holland, in 1705, with a Latin translation, as Historia Jeschuae Nazareni.
A summary of this work is included by Rev. Sabine Baring-Gould, The Lost and Hostile Gospels. An Essay on the Toledoth Jeschu, and the Petrine and Pauline Gospels of the First Three Centuries of which Fragments Remain. Williams and Norgate, London, 1874. A digital version of this work is available on-line at: https://archive.org/details/lostandhostileg00goulgoog
Although Blavatsky did not always commit herself to the 100BC theory, she did endorse it in several places, notably in 1887 in two articles:
“The Esoteric Character of the Gospels” was originally published in Lucifer in 1887 and is reprinted in H.P. Blavatsky Collected Writings VIII Theosophical Publishing House, Adyar, 1960:172-239]. The text is also available on-line at: http://www.theosociety.org/pasadena/hpb-sio/sio-eso1.htm
“Notes sur l’Esoterisme du Dogma Chretien de M. l’Abbe Roca“, Blavatsky’s response in French to the Abbe Roca’s “Esotericism of Christian Dogma”, was originally published in Le Lotus (Paris, December, 1887) and is reprinted in H.P. Blavatsky Collected Writings VIII Theosophical Publishing House, Adyar, 1960:355-371 (French version); 272-391 (English translation). Paul Roca (1830-1893) was priest of the Diocese of Perpignan in France, frequently in trouble with his ecclesiastical superiors as a result of his unorthodox teachings. See: http://www.cromleck-de-rennes.com/Abbe%20Roca.html
An English translation of Roca’s article is available on-line at: http://www.katinkahesselink.net/blavatsky/articles/v8/y1887_046.htm
An English translation of Blavatsky’s response is available on-line at: http://www.katinkahesselink.net/blavatsky/articles/v8/y1887_048.htm
A significant footnote is found in “Notes sur l’Esoterisme du Dogma Chretien de M. l’Abbe Roca“: Having drawn to Madame Blavatsky’s attention that, according to certain scholars, this assertion [i.e. that Jesus may have lived a century before the Christian era] is erroneous, she answered as follows: “I say the scholars and either lying or talking nonsense. Our Masters affirm the statement. If the story of Jehoshua, or Jesus Ben-Pandira is false, then the whole Talmud, the whole Jewish Canon is false. He was the disciple of Jehoshua Ben Perahiah, the fifth President of the Sanhedran after Ezra who re-wrote the Bible. Compromised in the revolt of the Pharisees against Jannaeus in 105 B.C., he fled into Egypt carrying the young Jesus with him. This account is far truer than that of the New Testament which has no record in history. [reprinted in H.P. Blavatsky Collected Writings VIII Theosophical Publishing House, Adyar, 1960:380]