Free will - the capacity for choice that is neither determined nor random but a property of personal will - is central to Christianity, it is necessary to Christianity.
A Christian must accept as axiomatic that there is such a thing as free will; he may seek to elucidate the concept, but that there is free will cannot be doubted, nor should empirical or rational justification for its existence be sought, since there cannot be any such justification.
It is a perilous misdirection of motivation to look for a justification which intrinsically cannot be found.
That humans posses free will is indeed common sense: the knowledge is spontaneous, ineradicable common sense; almost all of our behavior and social life is built on the assumption that free will is real.
The scope of free will, whether it is operative in a specific situation, the ways by which it is influenced by habit or coercion - such matters are legitimate topics for discussion or investigation; but that free will exists cannot coherently be challenged; and for Christians the fundamental operation of free will is a necessary component of the bedrock of reality.
Free will may therefore be considered an aspect of natural law; an aspect without which other aspects of natural law make no sense.
To doubt such axioms as free will is not a coherent strategy: all specific doubts must be based on (at least implicit, if not explicit) acceptance of axioms. Universal skepticism is rationally impossible. To doubt free will is simply incoherent.
However, precisely because free will is so fundamental a phenomenon, and even though we know what free will does, it is impossible to define with precision what free will is or how it works.
So the mechanism of free will, its inner causality, its strength, physics, psychology, neuroscience... such matters cannot be investigated nor analyzed.
Those who have a problem with free will, who are puzzled by it (which puzzlement has more of the nature of a pathology than of a legitimate topic for curiosity) are likely to be misled by the questions they ask.
The framing of specific questions that are intrinsically unanswerable leads, with a strong tendency, to misleading or plain false answers - because when a question is irrelevant the answer must be a non sequitur.
Curiosity based on mistaken formulation of the nature of free will cannot, therefore, legitimately be satisfied: at most curiosity may become distracted or simply exhausted.
Or the failure to satisfy personal curiosity or to allay personal dissatisfaction may be interpreted as some kind of refutation.
For Christians, free will is one of the essential attributes of Man. Christian salvation entails free will: it is only understandable on the basis that Man is a creature with free will.
Other attributes are necessary for salvation as well as free will; and it is assumed that other creatures than Man may also possess free will - for instance angels.
But Man's free will constrains the whole scheme and system of Christian salvation - without free will the human condition - Man's meaning, purpose and relation with God - is nonsense.
Therefore one cannot be a Christian and deny the reality of free will.
Of course what people say about free will, what people think they believe about free will, may not be an accurate description of their true beliefs.
But whatever he may say, or imagine, the Christian must truly believe in free will.
This in itself is not a problem - since there is no genuine alternative.
However confusions over free will may have the effect of blocking Christian conviction: when a person thinks that free will may not be true, or may not be coherent, or may be explained away in terms of (say) social pressure, psychology or neuroscience... then such doubts and worries may also block consideration of Christianity since it is correctly perceived that the Christian model necessarily entails the operation of free will.
Because understanding (even at the most basic level) Christian salvation, entails understanding that free will is intrinsic to Man.
Free will is not an optional extra or added bonus, but intrinsic to humanity and to individual destiny.
Reality only makes sense in terms of free will.
Why then does free will strike so many modern people as so vague and elusive and doubtful a concept?
The answer is that this is precisely what happens when humans try to deny the validity of their built-in nature and knowledge; when human challenge, subvert and invert natural law. When humans try to pursue a strategy of skepticism.
What results is not a deeper and more secure understanding - but no understanding at all, mere error and confusion, and if the skepticism is not recognized as a snare then this state of error and confusion may be permanent.
A culture like ours, which encourages, and indeed enforces, the systematic denial of all forms of natural law, will therefore permanently be unable to perceive or engage with reality.
To challenge, debate and 'investigate' the reality of free will as if it were a mere hypothesis is therefore a one-way door to nihilism: to implicit denial of the reality of the real.