Each deity has a set of mantras which define their core values, their personality, thoughts, desires, and actions. Mantras are paired, so if one adheres to a particular mantra, one will be opposed to the other, for example, chaotic and ordered. There are 18 mantra pairs.
One thing to keep in mind regarding these mantras and how they relate to a deity or character is how certain mantras would work in conjunction with others. For example, if you are cruel and selfish, then you would be sadistic. However, if you are cruel and selfless, then you would be masochistic. If you are ordered and active, then you would be the type of person to try to regulate others. However, if you are ordered and passive, you would be the kind of person to allow authority figures to regulate you. Chaos and active would be rebellious whereas chaos and passive would be flighty.
This system can be roughly conceptualized as an advancement upon the idea of alignment. Like alignment, mantras are intended as indicators of character (or deity) personalities and thus guides for role-playing a character in meaningful ways. Mantras, however, enable us to integrate such considerations into our game systems far more effectively than alignment. Thus, mantras go far beyond abstract representations of characters' (or deity's) outlook and ethics, or lack thereof.
The mantra system was conceived for the future. It is not applicable for NWN, although it gives players insight into the core ideals of each deity.