The concept of a creator God is constantly expressed through the series. While it's mostly disputed by humans, it's shown in some games that even non-humans, like the Mermaids, believe in a creator god, not to mention Angel Knights are sworn to said god.
Followers of Lodisism believe that said god is Filarhh, the sun god, and that all other god are His avatars. Followers of Philaha believe that He's the Father God, assisted by His six elemental daughters (Hahnela, Vaasa, Xoshonell, Greuza, Nestharot and Lyuneram). Followers of Roshfel maintain that their God was the one who led the forces of Heaven during the Ogre Battle, rules as patron god, and opposes the forces of the Netherworld.
Non-humans and demi-humans don't seem to have an structured doctrine of worship, holding a basic belief in their creator God and their role in the world instead. Angel Knights also don't specify whichever god they serve, limiting themselves to His orders and their mission.
The concept itself, and the religions that debate it, are heavily based on the Abrahamic concept of God.