Pythagoras

571 BCE – 496 BCE Greek philosopher, mathematician, founder of the Pythagorean school

Greek philosopher whose number mysticism and secret brotherhood shaped later esoteric thought.

Religion: Pythagoreanism, a religious and philosophical way of life teaching the transmigration of souls, the mystical order of numbers, and purification through study and discipline.
Affiliations: Founded the Pythagorean brotherhood at Croton, a closed community with secret teachings and shared rules of life.

Pythagoras founded a religious and philosophical community at Croton in southern Italy that combined mathematics, music, and mysticism with strict communal rules. His followers believed numbers held the key to the order of the cosmos and that the soul was reborn through many lives. Later esoteric writers, including the Renaissance Hermeticists and the Freemasons, looked back to him as a founder of hidden wisdom.

Connections

influenced Plato — Pythagorean number mysticism shaped Plato's theory of forms.
influenced Plotinus — Pythagorean number mysticism fed into the Neoplatonism Plotinus developed.

In the timeline

531 BCE Pythagoras and a theorem older than his name central figure
501 BCE Pythagoras founds his mystery-school academy central figure

Wikipedia