Press  the  key  above  TAB  (which is the tilde key) to open the
console.  Now  type  in  these  codes. NOTE: to test the invincibility
code,  shoot your team-mates (if you have friendly fire on), or toss a
grenade at your feet and stand on it.

Cheat code:             Description:
god                     Invincibility 
behindview              1 = 3rd person view
ghost                   No Clipping mode
fly                     Flying mode
walk                    disables ghost/fly