Choices typically feel like freedom. For most of us, it's case closed as soon as we say that. But from a neurolocigal standpoint, it's not so simple.
Sheer complexity in the brain means most of our mental workings are necessarily hidden from our conscious selves; like the iceberg, most of us is below the surface. When, confronted with a situation, we make we call a "choice," we do so because it seems to have come from nowhere; nobody forced us. Yet the states of the brain consist of chemical and electrical reactions.
To claim that we are somehow controlling those chemical and electrical reactions -- the mechanism of the brain -- means we have to have something else going on, and all that glorious biochemical mechanism is, well, irrelevant somehow.
That's not a problem for a traditional Christian, who just says, well, the spirit does the deciding, and the spirit is immaterial. But then, what are brains for, and why would construct them in such a way that chemical changes can be introduced that obviously not only our emotions, but our decisions?
What I'm arguing here is that just as we are products of nature, thought is a natural process, subject to natural law. We think we have free will because of complexity (if we can't clearly see the chain of causation, it's tempting to claim there isn't one) and because of the early indoctrination most of us have to dualistic thinking, both inside and outside religion.
Arguing that neural states are too complex and chaotic to predict does nothing to establish freedom, not in the sense of a true unconstrained agent that controls itself independent of those neural states. Freedom is a much more ambitious claim than unpredictability.