The sacred feminine refers to the representation of a mystical power of the earth, mother Goddess, is often symbolized through images or events connected with fertility. She would be deemed a pagan deity, she would represent fertility, family, life, birth, and healing.
Joseph Campbell was an American professor, writer, and orator best known for his work in the fields of comparative mythology and comparative theology. These were his areas of great knowledge and expertise. There are few that can compare to the knowledge base that he had.
In his now famous 1988 interview with Bill Moyers, The Power of Myth, starts as Campbell links the image of the earth, mother Goddess, to symbols of fertility and reproduction. He shows a form of unity between the people and the earth, as well as the people and the goddess.
The connection of the image of the earth, mother Goddess, to various rituals surrounding, and images of, fertility was (and still is) common to numerous cultural and religious traditions including Gnosticism.
For example, Campbell states that, "There have been systems of religion where the mother is the prime parent, the source...We talk of Mother Earth. And in Egypt you have the Mother Heavens, the Goddess Nut, who is represented as the whole heavenly sphere."
Campbell continues on by stating that the correlation between fertility and the Goddess herself may lay in the connection to agriculture itself. That perhaps the goddess represents the sowing of grains and other crops for a good harvest year. That perhaps the growing year and a good harvest would represent her favor in her followers.