During gameplay press the '~' (tilde) key to bring up the command console then type the following codes (without the quotes) to enable the corresponding effect.
Type 'cvarAdd g_permaGodMode 1' (or try simply typing in 'god')
Type 'cvarAdd g_permaInfiniteAmmo 1'
Type 'cvarAdd g_inhibitAi 1'
Type 'DebugUnlockPerkByAbility -1'
Unlock Perks 2:
Type 'DebugUnlockPerkByRequirement -1'
Type 'g_fov X' (where X is the FOV number)
Developer Mode (Disables Steam Acheivments):
Type 'dev 1'
Type 'Com_showFPS 3'
Easy 'Argent Fiend' Achievement:
An easy way to get this Achievement is to progress in the Single-Player Campaign until you reach the final Boss and then quit and replay the missions to get the collectables.