In-game press the ` tilde key to bring down the console and enter one of the following codes (use 0 to toggle off and 1 to toggle on):

Code - Effect

god - God Mode
aiDisableEars - Enemy AI cannot hear you
aiDisableEyes - Enemy AI cannot see you
noclip - Walk through walls
aiGodMode - Enemy God Mode
aiProtectProtectee - Protectee God Mode