After collecting the 4 kinds of Secret Stones, click on the King of your Kingdom while you are wearing Sephirot(any kind) and Eternal Stone, and the Ideal Stone will appear in your inventory. (Possible for God and Celestial God only).
Use Ideal Stone to change a God into a Celestial God . However, your mortal character must be at level 400 and your God must be level 356.
[Ideal Stone]