Open console (Tilde "~" for QWERTY keyboards or key "%" for AZERTY keyboard).
Enter: Kingsoft 1 to activate cheat Codes. Then you can use:

Cheat      Effect
------------------
God 1    - God mode.
God 0    - Disable God mode.
FullLife - refill Max Health.
fly 1    - Fly mode (useful to pass through walls or others obstacles).
fly 0    - Disable flight mode (fly 0 for normal mode again).
Reborn   - ???