"The sender and sent are one in the same."
The goddess, becoming defiled, causes the Seethe to appear. When Soh and Yoshiro are purified completely, there's no more Seethe and no more cycle. Soh and Yoshiro are the goddess split into two beings.
This I'm pretty sure of. To better explain, Yoshiro takes on the role of the Jesus figure and Soh the Holy Spirit. But they're all aspects of the Goddess herself that descended unto mankind to help them.