Beat the game on Fetish difficulty, then at the Title screen press Down, Down, B, Up, X, Y, X, White, Up, Up, L, Y. You can't save the game in God mode.
To enable the following codes, you must first beat the two levels listed for each code on Hardcore difficulty. Then enter them at the Start menu:
R, White, L, Black, Down, Up, Left, Up. Beat Drunk Driving and Graveyard Shift levels first.
X, X, X, Down, X, Down, B, Up. Beat Trained To Kill and Border Patrol levels first.
White, Right, B, White, Black, Down, B, Left. Beat Fueled By Hate and Grounds For Assault levels first.
L, Y, Y, Y, B, B, B, R. Beat Mouth Of Madness and Doing Time levels first.
R, L, White, L, Right, Left, Left, Left. Beat Road To Ruin and White Trash levels first.
White, White, L, White, Left, Right, Left, Right. Beat Born Again and Doorway Into Hell levels first.
X, X, White, Down, Y, X, B, Down. Beat Press Coverage and Wrong Side Of The Tracks levels first.
Left, R, R, Y, R, R, X, L. Beat Kill The Rabbit and Divided They Fall levels first.
Up, Down, Left, Left, R, White, L, L. Beat Key Personel and Deliverance levels first.
Thanks to Revolution readers Shoemaker, chuck, phly and wonde!