PC
While playing, press "~" to bring down the console. Type "iamtheone" to activate cheats. Then type "god" for God Mode.
Flight Mode
While playing, press "~" to bring down the console. Type "iamtheone" to activate cheats. Then type "fly" for Flight Mode.
Ghost Mode
While playing, press "~" to bring down the console. Type "iamtheone" to activate cheats. Then type "ghost" for Ghost Mode.