Most people think that mediumship is concerned with proving the existence of the afterlife, usually by reuniting people with their crossed-over loved ones.

Mediums (sometimes called 'prophets') have always been channelling philosophy - teachings about the purpose of life, the nature of the afterlife, and spiritual evolution.

From the beginning of the 1800s, we have records of mediums who channeled collective groups of high-level spiritual teachers, some of whom lived thousands of years ago. It was claimed that they sought to convey important messages to humanity to counter the growing influence of materialism.

ALAN KARDEC AND SPIRITISM

Alan Kardec (Hippolyte Léon Denizard Rivail) claimed that "The Spirits' Book" (Le Livre des Esprits), published in 1857, was written based on communications he received from various spirits through mediums. According to Kardec's account, the primary spiritual influences included:

The Spirit of Truth - Presented as the primary guiding spirit and highest authority
Saint John the Evangelist - Claimed to provide Christian theological guidance
Saint Augustine - Said to offer philosophical and moral teachings
Socrates - Attributed with contributing philosophical insights
Plato - Also cited for philosophical content
Fénelon - The French archbishop and theologian
Swedenborg - The Swedish mystic and theologian

WILLIAM STAINTON MOSES

William Stainton Moses (1839–92), an English clergyman and physical medium, channeled an organised band of 50 spirits between 1872 and 1883. His work was published in Spirit Teachings and More Spirit Teachings. The identity of the communicators was only gradually disclosed. Eight of them were known characters from the Bible including Malachias, Elijah, Haggai, Daniel, Ezekiel, and St. John the Baptist. There were also fourteen ancient philosophers and sages, including Solon, Plato, Aristotle, Seneca, Hippolytus, and Plotinus.
They claimed that the spirit realms were conducting a missionary effort to uplift the human race. More details on the communicators.

SILVER BIRCH

Maurice Barbanell's Silver Birch claimed that he represented "the voice of the spirit that seeks to make its presence felt in your world and which is succeeding in increasing measure.” He described
“a vast concourse [that] belongs to all peoples and nations, to the present and to the past. There are prophets, seers, sages, wise men of the East and the West, of high and low estate, philosophers of Greece and Rome, Syria, Chaldea, Persia and Babylon, mingled with those of later generations from Italy, France and Germany. They exchange their knowledge and focus it all so that it shall be at your service. All of them were “servants of the Great Spirit. Some of us have evolved a little higher. Because of that, we return to give service, for service is the law of life.” See "Who Was Silver Birch".