The Essenes were an ascetic Jewish sect who lived near the Dead Sea around the time of Jesus. Up until the 20th century, very little was known about them by historical scholars. The sources were only some scattered remarks by the ancient Jewish writers Philon and Josephus, and the Roman writer Pliny the elder. This lack of knowledge made the mysterious Essene sect a suitable vessel for all manner of wild speculations and fantasies -- which is exactly what happened.
In the occult undercurrents of Western society, there is a pattern of ideas about history called "the Essene theory". This goes back to the early 18th century when radical "enlightenment" and rationalism had caught on in England. Suddenly there was no room for mysteries. Everything had to be rational. Miracles, angels and resurrections from the dead are not reasonable, so many thinkers wanted to get rid of those parts of the Christian faith. Great ingenuity was applied to invent "natural" explanations for such scriptural passages. Parallel with this rationalism, the 18th century was a heyday for secret societies and orders, which supplied some of the mystery and romanticism that were no longer politically correct in society. The idea popped up that perhaps the "natural" secret about Jesus was that he belonged to a secret society which had superior (scientific) knowledge?
A Prussian theologist named Karl Friedrich Bahrdt was the first to publish a theory based on these ideas. He considered the biblical accounts of Jesus' life unreasonable and suggested instead that Nicodemus and Joseph of Arimathea were Essenes, an order devoted to raise people's minds to a higher knowledge. Jesus and his disciples were received into the order, but the disciples were not initiated into the highest degree and consequently never understood Jesus' teachings correctly. Jesus survived the crucifixion, and retired from public life. So far Bahrdt. Next, a pastor named Karl Heinrich Venturini embellished Bahrdts story in a 4-volume story of the life of Jesus. These books later inspired several novels about Jesus.
The author of the Essene epistle summarized and abbreviated these novels into letter format, and presented it as a manuscript find. The "Essene epistle" first appeared in German, printed in Leipzig 1849 and financed by a "German Brotherhood" which was probably a Masonic lodge. Freemasons on the European continent liked to see themselves as the true inheritors of the Essenes. The epistle expresses precisely the kind of combined rationalism and mystique that Masons and other secret societies were espousing at the time.
The epistles' contents are pretty much in accordance with the "Essene theory" described above. The rituals for making Jesus a member of the secret Essene order are described. Jesus did not die on the cross, he was almost dead but Joseph of Arimathea healed him with the secret medicines of the Essenes. What the Bible describes as angels at the transfiguration, the grave and the ascension were actually Essenes in white robes. The disciples did not know the Great Secrets and falsely believed that Jesus was resurrected from the dead. Jesus actually died 6 months after Easter.
The epistle may have been credible as a genuine manuscript find in the 19th century, but the finding of the Dead Sea scrolls in 1947 has definitely put an end to any such possibility. A comparison demonstrates that the "Essene epistle" cannot be a genuine document from the Essenes. Just a few points to make this clear:
The epistle is clearly a hoax. Its influence has nevertheless been enormous. New editions have been printed in various languages time and again. It is used by the half-islamic Ahmadiyya movement as evidence for some of their beliefs. Several later hoaxes, e.g. "Jeshoua the Nazir" and "The Gospel of Peace" are hardly imaginable without the epistle as model.