The major proponent of the theory of heliocentrism during the Renaissance was Nicolaus Copernicus. His groundbreaking work, "De revolutionibus orbium coelestium," proposed that the Sun, rather than the Earth, is at the center of the universe, challenging the long-held geocentric view. This theory laid the foundation for modern astronomy and ultimately influenced later scientists, such as Galileo Galilei and Johannes Kepler, in their discoveries, marking a significant shift in scientific thought during the Renaissance.