Enter the Cheat screen code from the main menu to call up the cheat screen. From here you can enter any level and turn on God mode and infinite ammo. Enter the other codes from the main menu to give yourself those abilities. You will hear a clicking sound to confirm the cheat is active.

Code - Effect

48662 - All weapons
24328 - Cheat Screen
48627 - Infinite clips
46266 - Infinite ghost vision
67687 - Unlock all single player maps