Free will is absolutely central to Christianity - but what kind of a thing is free will as conceptualized by Christians?
It is tricky, because it does not seem to be like anything else - or at any rate I find it hard to find an analogy for free will.
Its properties must include on the one hand an ability to choose, whatever past experience a person has had - yet on the other hand free will must be (and clearly is) influenced by past experiences.
So, free will is, in one sense, an unchanged changer, persisting throughout life; yet in another sense its current choices are affected by previous choices - such that in some sense the probabilities of choices change throughout life (good habits/ virtues practiced over long periods make it more likely the next choice will also be good/ virtuous - and vice versa).
The will must be free, and yet it is clear that the human will is corrupt (this is explained by original sin; but the fact that the human will is corrupt is a matter of observation and experience).
Human have free will, yet will almost always choose wrong; and we are - indeed - helpless to do otherwise; which is why we cannot save ourselves but must have a Saviour.
On the one hand we are saved by Christ - because we cannot save ourselves - yet we are also required to choose, despite that we will inevitably, helplessly choose wrong most of the time.
How to make sense of this?
I think only by assuming that or free will works about other people but not ourselves; our choices can save others but not ourselves.
And the primary choice is to love not hate others; to be loving and humble not prideful towards God.
I think this is pretty much all that most people (nearly everybody) can do with free will.
And I think this is what is implied by the two main commandments from Matthew 22 as interpreted in the Book of Common Prayer:
37 Jesus said unto him, Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind. 38 This is the first and great commandment. 39 And the second is like unto it, Thou shalt love thy neighbour as thyself. 40 On these two commandments hang all the law and the prophets.
This would mean that the main function of life on earth is to love first God, then neighbour - but that it is the loving of neighbour which requires mortal life on earth, and therefore is the primary function of free will.
Most people cannot live by the Law, cannot reform themselves or even their external lives - but what they must will is described in these two commandments.
And love of neighbour is a matter of will, not action - action is constrained, will is not.
And it is never too late, and we are always free to do this, and nothing whatsoever (either internal or external) stops us doing this, and no amount of experience or habit matters, it can be overturned in the blink of an eye - love of neighbour is the proper domain of free will.
Thus we can, by our love for them, save others (but not ourselves).