Psychology, being a fairly new disciplin among the great sciences, is still in its infancy. What it cannot yet predict, it has not yet had the chance to test or examine. The potential is there to predict as much social behaviour (how we make choices, how we are persuaded, who we’re attracted to, what types of people will subscribe to religious experiences, etc.) as brain-related cognitive behaviour (memory, attention, perception, language, problem solving, etc.). It’s not that psychology can predict all behaviour (it doesn’t have all the variables with which to do it), it’s that it shows just how predictable behaviour is within impressive confidence intervals. This is, whether we want to acknowledge it or not, a threat to the idea of free will. I never said that behaviour was just biological; it is, as you say, nurture (but I call this environment, not the soul). Psychologists that reference the soul in true Christian terms are few and far between. The interaction psychologists are often looking at is environment and biology, not biology and soul/mind. As I have suggested though, the threat is indirect - it’s really only there for whomever would like to raise the argument. Mostly, Psychology just doesn’t invest itself in the metaphysical.