Often, unstudied Christians (I speak from experience) misunderstand the questions others ask concerning free will.
To the Christian, free will often raises the thought of issues related to man's ability and responsibility when it comes to original and actual sin.
However, for the naturalist / nihilist, while the issue can involve matters related to ability & responsibility (i.e., ethics), it's often more a question of being and meaning, that is... if matter is all there is, and if the cosmos operates with a uniformity of cause and effect in a closed system [some differ on this], and if man is more or less a machine, then the question among naturalists & nihilists when it comes to "free will" deals more with significance, dignity and meaning.
James Sire in his book The Universe Next Door puts it well when he says:
"The issue of human freedom goes deeper than these naturalists see... [SURE] I can do what I WANT, but WHAT I WANT is the RESULT of PAST STATES OF AFFAIRS over which ultimately I HAVE NO CONTROL. I did not freely select my particular genetic makeup or my original family environment. By the time I asked whether I was free to act freely, I was so molded by nature and nurture that the very fact that the question occurred to me was determined. That is, my SELF ITSELF was determined by outside forces. I can indeed ask such questions, I can act according to my wants and desires, and I can appear to myself to be free, but it is appearance only. Nietzsche is right: 'the acting man's delusion about himself, his assumption that free will exists, is also part of the calculating mechanism.'" [CAPS, MY emphasis]
Christians in debating unbelivers over free will must be careful to understand and address what's actually being asked and/or what's actually being addressed, for what's at hand on one level goes deeper than the relationship between man's "free and bound" will even to the issue of the existence of free will.