Hold [Left Shift] and press [Keypad Minus] at the indicated screen. Release the keys, then type one of the following codes to activate the corresponding cheat function.
Effect - Screen - Code
God mode - Any - iwillbetheone
Saves game - Game play - savegame
End current mission - Game play - endmission
All missions unlocked - Main menu - campaign
Show elevations on map - Main menu - topography