As long as there have been books, people have been fascinated by the dream of discovering an old, unknown book with sensational or illuminating contents. Already in ancient Egyptian, Greek and Roman literature there are lots of stories about books or letters which have fallen från heaven, been found in graves, temples and libraries, and contain magical recepies, revelations or oracles. It certainly happened then, like today, that real old manuscripts were found. But the few authentic finds are a drop in the ocean compared to the amazing number of hoaxes and fakes.
In the hundred or so years after the New Testament, there was a flourishing popular genre of scriptures known as apocryphs. Miraculous stories about apostles, letters by apostles, and apocalypses -- all claiming to have been written by this or that apostle. They were written significantly later than the New Testament, a fact which the church recognized when the Biblical books were collected into a canon. The apocryphic books were not included in the canon but were accepted as a pious folk literature -- not historically true, not inspired by God, but of some spiritual value in the same way as a Christian novel may be today.
Other apocryphs are heretical from a Christian point of view. They are inspired by the ideologic fad of the time, gnosticism, and are actual attempts to deceive people into believing that Jesus had proclaimed a gnostic message. The "Thomas gospel" may serve as example. Modern manuscript finds of the Thomas gospel bear out that the Church was correct in proclaiming it to be a pack of gnostic fantasies about Jesus.
In the Middle Ages, a new phenomenon appeared: the "heavenly letters" which were thought to have fallen from heaven and purported to have been written by God himself or by Jesus. They can perhaps be seen as an equivalent of the "prophecies" often occurring in evangelical-charismatic Christian churches today. A result of prayer and pious thoughts but not intending to say anything radically new and different. At the Renaissance, antiquity was seen as the great golden age and the interest in ancient documents increased drastically. From this time, hoaxes are increasingly accompanied by a story of how the manuscript was found, and other supporting side material. The most famous Renaissance hoaxes are the "Barnabas gospel" and the "Death sentence of Jesus".
In mid-19th century professor Constantin Tischendorf made his famous discoveries at the St Catherine monastery in Sinai -- genuine finds of very old Biblical manuscripts which had been hoarded by the orthodox monks but remained unknown to the outside world. The news about these finds obviously stimulated imaginations, because suddenly the number of hoaxes exploded. This was also the time when scholars began examining the Bible critically, treating it like any other ancient text. They applied scientific methods and disputed e.g. whether king David could really have written the psalms (which mention praising God in the Jerusalem Temple even though no temple existed there in David's lifetime), and so on.
19th century hoaxes differ from earlier pseudo-biblical documents, in that
The fashions and fads of the time are clearly reflected in the hoaxes. At this time there was a vast new popular interest in India and the romantic, mysterious East. Sure enough, Nicolas Notovitch "disovered" his manuscript in Tibet and we soon get hoaxes which claim that Jesus went to India. Levi Dowlings "Aquarian Gospel" is another Eastern hoax, where Jesus preaches reincarnation. Joseph Smith decided to invent Mormonism as a new American religion; sure enough, his writings claim that Jesus went to America after the resurrection.
Unexpectedly, many genuine scriptural finds were made in the 20th century: the Dead Sea scrolls, the gnostic library from Nag Hammadi in Egypt, the Jewish fragments from Masada, and even more books from that old manuscript treasure trove, the St Catherine monastery in Sinai. The hoaxes have followed the same pattern as in the previous century, imitating the genuine finds. So, for instance, those parts of the "Gospel of Peace" which were published after the Dead Sea finds, are clearly inspired by them.
20th century hoaxes cater to modern religious and philosophical fads just like hoxes always have. Many popular 20th century ideas have been put in Jesus' mouth -- environmentalism, feminism, socialism, UFO-salvation, vegetarianism. With the better scholarly and scientific methods available for manuscript examination, hoaxers have been forced to become more sophisticated in order to convince people that their forgeries are genuine. Conceivable counterarguments are anticipated and and explained away in the "find story" or other material that accompanies the text.
Everything indicates that the idea of "the unknown Jesus document" is deeply ingrained in many people's subconscious, hoping or fearing that such a book will be discovered, while at the same time it can be filled with anything that happens to be on people's minds. In all probability, new ideologies and future debates will seek support in new forgeries.