NOTE: I have already had second thoughts about this post! More to come on this topic. But it may be worth seeing my 'workings' and the comments by Nathan, for those who find Free will 'a stumbling block'.
Free will is an intrinsic part of Christianity - there can be no Christianity without Free will.
Yet Free will is a metaphysical assumption, not a discovery - no experience, no evidence can make any difference to the reality of Free will: either it is assumed or it is not.
But what is it? How does it work?
Ultimately, the question is bogus, and did not trouble our (most intelligent and thoughtful) ancestors; the question is unnecessary, unanswerable, pseudo-profound...
But why does the question nonetheless torment some people spontaneously?
How can the question be dealt-with; can anything be done to clarify matters?
One reason for this question may be that we spontaneously see Free will as a little man inside our heads who chooses between alternative courses of action: a homunculus controller sitting at a console in our brains pressing button A or button B.
Obviously this is a grossly simplified metaphor, but it is hard not to think along such lines.
Is there an alternative metaphor for free will? One which captures its essence to some extent - rather than merely kicking the can down the road from ourselves to another but smaller person doing the choosing?
Perhaps the answer is to break out from linear, sequential causality and to try the alternative of field thinking - along the lines of Rupert Sheldrake's version of morphic fields which organize and impose form.
If free will was conceptualized as an organizing field - perhaps something like a magnetic or gravitational field - it could be seen as exerting its influence by a tendency to impose a pattern on human behavior (as a magnet imposes a pattern on iron filings).
As a field, Free will might vary in its strength and range, and might interact with the consequences of its own action (as a magnetic field attracts iron, and the closer the iron moves the stronger is the effect of the magnetic field).
As yet the metaphor of Free will as an organizing field is vestigial - but promising enough that I think it could perhaps be helpful in breaking-out from the apprent inevitability of a homunculus controller.