Jacques Roger Jean Breyer (27 March 1922 – 25 April 1996) was a French esotericist, alchemist and writer. He published and wrote various books on esoteric elements, including ones with apocalyptic teachings. He launched the "Arginy Renaissance", a rebirth of an independent wing of neo-Templar groups — groups that claimed to be revivals of the Knights Templar — in France in the 1950s. He was influential on the development of many of these organizations, including the Order of the Solar Temple.

Jacques Roger Jean Breyer was born 27 March 1922 in Noyon, Oise, in France. He was raised in Anjou, born to a family of wine sellers. His secondary education was interrupted by the outbreak of the Second World War; he joined the French Resistance, where he was a second lieutenant helping the Comet Line. He was arrested by the Gestapo in April of 1944, and then deported to a concentration camp in Buchenwald and then Flöha. While there he met and befriended writer Robert Desnos.

Following the liberation of France, he was ill with tuberculosis – visiting the Pyrenees for treatment, his mother gifted him a few items that had belonged to her father, one of which was a grimoire of herbal medicine. Following this, he became interested in esotericism.

In 1951, he contacted the owners of the Castle of Arginy, trying to interpret a possible esoteric meaning to the graffiti on the keep walls. The following spring, he would move into the farmhouse next to it, where he would live for the next seven years; while living there, he would care for its animals and the sick, lecture in Lyon and Paris, and perform theurgy in its dungeon.

That year he met a journalist attempting to write an article on the site, Marcel Veyre de Bagot, who he befriended and attempted theurgy alongside. Breyer viewed this as the start of a rebirth of the Knights Templar (later called the "Arginy Renaissance"), the medieval Order of the Temple. Breyer's actions in Arginy lead to a second, independent branch of neo-Templar groups (compared with the main OSMTJ organizations that had existed for many years).

Breyer began to acquire a circle of freemason and occultist orbiters, including Armand Barbault, Jean de Foucault, Victor Michon, Vincent Planque, Pierre de Ribaucourt, Maxime de Roquemaur, and Jean Roux. Later inclusions were Jean Soucasse, then Robert Chabrier, Bernard Wiernick, and Georges Sourp. Roquemaur claimed he was a descendant of a branch of the original Templars, who had secretly survived in Ethiopia, carrying on a supposed esoteric legacy from these original Templars. Roquemaur and Breyer then proceeded to found the Sovereign Order of the Solar Temple (OSTS), members of which founded several other Masonic organizations in France.

In this period, he wrote his first two books, Dante Alchimiste in 1957 and Arcanes Solaires: Ou les secrets du Temple Solaire two years later. Breyer claimed that he had discovered a document dated to the 1700s in the French BnF, which allegedly stated that the final Grand Master of the Knights Templar, Jacques de Molay, had passed on his authority to his nephew; further, he identified Arginy as the original location at which the Templars had been founded. He said that relics would be found at the family estate of this nephew – no relics were found, but Breyer started workshops in Ergonia which drew the interest of others.

He claimed these writings related directly to knowledge possessed by the original Knights Templar, though he later denounced these works as "incomplete" and "inaccurate". The OSTS's ideology focused on heavily apocalyptic elements, including the "Solar Christ" concept. Breyer claimed that he had founded the OSTS at the suggestion of the "Masters of the Temple", who he said were the spirits of the Knights Templar. Breyer would become greatly influential on subsequent neo-Templar groups and was viewed as a spiritual mentor by many in them. Breyer resigned from the OSTS in 1964, after which the OSTS was embroiled in crisis; they reorganized again in 1966 and 1973.