Press ~ to open the console. Type in the following and press Enter to activate. 

thetrooper           God mode 
stilllife            Invisible 
sunandsteel          Reload weapons 
diewithyourbootson   Unlimited ammo 
revelation           8 Artillery rounds 

Start the game with one of these command lines to activate cheat.

/s               Enable Save Scores Mode
/r               Record Demo 
/?               Command Line Help
/d               Read Resources from Directory
/p[file name]    Playback Demo