I will be using a book of the Italian historian Barbara Frale, who is as of currently a researcher at the Vatican Secret Archive, "I Templari", "The Templars", Il Mulino, Bologna, 2007. I am not aware if this text has been translated in another language than Italian.
As it seems, most occult and supernatural connections which have been made with the Templars are result of myth and falsehood. I will elaborate.

The first and foremost is the alleged connection with the Baphomet, believed to have been a sort of idol or statue which was venerated instead of Christ. XIX century orientalist Joseph von Hammer-Purgstall claimed that the Baphomet was the idol of a calf obtained by the interpretation of a certain hieroglyph which would read "Bahumid", thus linking them with ancient Egyptian magic and sorcery and the secrets of the Pyramids. To be thorough, the term "Baphomet" appeared in the documents of the trial of the Templars in Carcassonne of 1307, alongside to the word "Yalla", as some inquisitors and officials wanted to fabricate a charge of heresy and apostasy, since they had either started occult practices or converted themselves to Islam. The latter appears to be the most plausible, since both Baphomet and Yalla look like mangled terms for Muhammad and Allah, but as Muslim teachings go, it is not possible to represent the face of the prophet and thus the presence of statues and idols of Muhammad was very, very unlikely.
A document from 1308, the "Chinon parchment", recounts a series of witnesses and accusations made by pope Clement V himself. 2 out of 1.114 testimonies have the word baffomettum in them, and 1.061 Templars out of 1.114 claimed that were no idols of any sort within the Order (as a clarification, most of these Templars were stationed in Europed and worked as either labourers or clerks in the many establishments owned by the Order). It must be said that by the time of the trial the Order had become very decadent and plagued by ill customs (some postulate that some French Cathars may have joined it and also the Hospitallers to escape excommunication), but pope Clement V absolved them of all charges in the same year.

There can be made a division between the history of the Templars (which ends 1314, when the Order stops working as a religious entity) and Templarism (which is the collection of myths built around them and after that date). During the Englightenment, the myth around them had two declinations: those who saw in them the forefathers of truth, reason and freedom of worship just like the many secret societies of that time (ironically, Adam Weishaupt's Illuminaty started in the late 1700s as an association of intellectuals and scholars, hidden away from censorship and repression), but also as the first sympton the the decay and derangement that these same ideals would eventually bring, corrupting both moral and religious principles.
In 1818 Hammer-Purgstell published his Mysterium Baphometis revelatum, ("Discovery of the Mystery of Baphomet), claiming they were secret continuators of the Early Christian Gnostic sect of the Ophites shifting the etymology of the word Baphomet to Greek, a theory already proposed by librarian Cristoph Friederich Nicolai who knew the strife brough by these Gnostics to the early Christology and theology. This thesis was already circulating in Europe since the latter century, summarized in Gotthold Ephraim Lessing's 1778 " Apologie oder Schutzschrift für die vernünftigen Verehrer Gottes ("An apology for, or some words in defense of, reasoning worshipers of God"), an unpublished volume of Hermann Samuel Reimarus which negated the divinity of Christ and saw him just a political leader of the Jews.
These writings and theories were born of the cultural and intellectual position of anti-Catholicism, made by scholars of languages, philosphy and antiquities, placed in an effort for the construction of free of the dogmas of the Church.

The latest mythography around the Templars as keepers of an original truth of the Christendom does come from the books and novels published between the 1980s and 1990s, first the book The Holy Blood and the Holy Grail, written by Michael Bagent, Richard Leigh and Henry Lincoln in 1982 and of course, Dan Brown's Da Vinci's Code, which capitalized on this new "shape of Jesus" as more human and less ethereal.
However, in 1945 near Nag-Hammadi in Egypt a whole library of Gnostic texts has been found (they have been translated and are readily available), which has indicated the source for this later aforementioned myth building. Templars and Gnostics could not have warded the secret of Jesus' marriage and birth of his daughter because the Gospel of Philip and the doctrine of these Gnostics preached sexual abstinence and ascetic practices, regarding marriage literally as faeces - garbage needed to survive. Plus, Jesus was not made by flesh, impure and corrupt, but only of divine spirit, disregarding even his death on the cross as a sort of hallucination.

Sorry for the lengthy answer, I hope to have cleared the historical perspective on the matter.