No blood:
Pause the game, then press Up, Down, Left, Right, Up, Up, Up, Up. 

Unlock all comics:
Pause the game, then press Up, Down, Left, Right, Right, Left, Left, Up. 

Unlock encyclopedia:
Pause the game, then press Up, Down, Left, Right, Left, Right, Up, Down. 

Level select:
Pause the game, then press Up, Down, Left, Right, Left, Left, Right, Right. 

All weapons:
Pause the game, then press Up, Down, Left, Right, Left, Right, Left, Left. 

Unlimited ammunition:
Pause the game, then press Up, Down, Left, Right, Up, Left, Down, Right. 

Unlimited Necroplasm:
Pause the game, then press Up, Down, Left, Right, Down, Left, Up, Right. 

Note: Spawn will speak to confirm correct code entry. 

Easy Souls:
On level 7 (Power Behind The Throne), fight all the demons and leave 
through the exit door. Then, turn around and go back. 
All of them will be back.

Defeating the Super Redeemer:
When fighting the last Boss, the Super Redeemer, use the Brimstone Cannons. 
When you hit him, he will drop ammunition for various guns.