To be sure, Hume stated his case too strongly. Reason isn't necessarily the slave of the passions. And we don't think it ought to be all of the time. But modern research has shown that cognition and emotion are interwoven systems, with emotion often taking the lead in helping us think correctly and virtuously.
This is a bit different from how the Greeks viewed the situation. For the Greeks emotion was error-prone and wild. Consequently, the wise person would use reason to subdue, tame, and guide the emotions. Thus the vision of the detached, cool, and cerebral philosopher.
We now know that the Greeks got this wrong. When emotion is decoupled from reason we have something that looks like sociopathy. At the very least reason needs emotion to do its work properly. I'm thinking here of work done with persons with damage to the ventromedial prefrontal cortex. People with damage to this area of the brain have trouble connecting emotion to how they make decisions or plans. Because of this these individuals can make long pro versus con lists but never reach a final decision. Cognitively these individuals have the ability to plan, and in great detail. But without emotion the cognitive system doesn't care about one outcome over others. And this caring, this emotional attachment, seems to be what breaks the rational stalemate and terminates the chain of calculation. In this reason is functioning as the servant, if not the slave, of the passions. It's as if Reason is saying to the Emotions, "Hey, I'll do all the calculation and accounting, but at the end of the day you're going to have to tell me what we really care about."
It is as Hume once provocatively argued. He said, "'Tis not contrary to reason to prefer the destruction of the whole world to the scratching of my finger." There is nothing unreasonable or illogical in preferring scratching your finger over preventing the destruction of the whole world. Yes, such a choice is monstrous and evil, but it's not illogical. The monstrosity goes to the issue of caring and emotion. What is broken in preferring scratching your finger over preventing the destruction of the world isn't reason, but emotion.
What does this have to do with theology?
Simply this. When theology and doctrine become separated from emotion we end up with something dysfunctional and even monstrous. A theology or doctrinal system that has become decoupled from emotion is going to look emotionally stunted and even inhuman.
What I'm describing here might be captured by the tag "orthodox alexithymia." By "orthodox" I mean the intellectual pursuit of right belief. And by "alexithymia" I mean someone who is, theologically speaking, emotionally and socially deaf and dumb. Even theologically sociopathic.
(Alexithymia--etymologically "without words for emotions"--is a symptom characteristic of individuals who have difficulty understanding their own and others' emotions. You can think of alexithymia as being the opposite of what is called emotional intelligence.)
Orthodox alexithymia is produced when the intellectual facets of Christian theology, in the pursuit of correct and right belief, become decoupled from emotion, empathy, and fellow-feeling. Orthodox alexithymics are like patients with ventromedial prefrontal cortex brain damage. Their reasoning may be sophisticated and internally consistent but it is disconnected from human emotion. And without Christ-shaped caring to guide the chain of calculation we wind up with the theological equivalent of preferring to scratch a doctrinal finger over preventing destruction of the whole world. Logically and doctrinally such preferences can be justified. They are not "contrary to reason." But they are inhuman and monstrous. Emotion, not reason, is what has gone missing.
(In my opinion, hard-core, double-predestination Calvinism looks just like this. An icy, monstrous and alexithymic theology.)
In their defense, the orthodox alexithymics will emphasize the view of the Greeks: reason must tame the passions. We cannot discern the will of God if we allow our feelings to get in the way. Emotions are temptations. Therefore we must make our feelings submit to reason. Reason leads you toward God. Emotion leads you away from God. So put your feelings to the side. If a chain of theological reasoning starts to horrify you then you must repress those feelings. Stuff that horror, swallow it.
But in light of what we now know about the relationship between cognition and emotion this Greek-inspired defense is sounding more and more hollow. And dangerous. A theology that is repressing the emotions, we suspect, just like in other spheres of life, is more rather than less likely to lead us astray.
Theology, as an activity of reason, might not want to be a slave of the passions, but it might want to partner with emotion much more closely.