ActRaiser takes place several years after God of War 3The Evils of Pandora's box merge together and become Tanzra, who, out of some sick idea of fun, decides to take the souls of humanity and recreates the world into his own personal playground. The Master is the metaphysical embodiment of Hope released from Kratos' body destined to bring an end to the chaos Tanzra wrought and restore the world to life, by any means necessary.
There's a reason the bosses sometimes look like "good" godsThe horrible demons and monsters being empowered by Tanzra/Satan are taking on the guise of good gods like Ganesha or Kali to both trick people into worshiping them and to weaken any ACTUAL other gods as well as the player by shifting the focus of prayer-power.
The Master in this game is the same Master as the Master in SoulBlazerLets see here the game is made by the same company with a very similar theme of a godly being trying to save the people of the world and by trying to rebuild the ruined world. Perhaps SoulBlazer takes place well after this game and the Master is now letting someone else fight the monstrous forces of evil.