From Surprised by Joy The Moment with a choice of salvation:
The odd thing was that before God closed in on me, I was in fact offered what now appears a moment of wholly free choice. In a sense. I was going up Headington Hill on the top of a bus. Without words and (I think) almost without images, a fact about myself was somehow presented to me. I became aware that I was holding something at bay, or shutting something out. Or, if you like, that I was wearing some stiff clothing, like corsets, or even a suit of armour, as if I were a lobster. I felt myself being, there and then, given a free choice. I could open the door or keep it shut; I could unbuckle the armour or keep it on. Neither choice was presented as a duty; no threat or promise was attached to either, though I knew that to open the door or to take off the corslet meant the incalculable. The choice appeared to be momentous but it was also strangely unemotional. I was moved by no desires or fears. In a sense I was not moved by anything. I chose to open, to unbuckle, to loosen the rein. I say, "I chose," yet it did not really seem possible to do the opposite. On the other hand, I was aware of no motives. You could argue that I was not a free agent, but I am more inclined to think that this came nearer to being a perfectly free act than most that I have ever done. Necessity may not be the opposite of freedom, and perhaps a man is most free when, instead of producing motives, he could only say, "I am what I do."
From That Hideous Strength - The Moment with a choice of damnation:
Still not asking what he would do, or why, Frost went to the garage. The whole place was silent and empty; the snow was thick on the ground by this. He came up with as many petrol tins as he could carry. He piled all the inflammables he could think of together in the Objective Room. Then he locked himself in by locking the outer door of the ante-room. Whatever it was that dictated his actions then compelled him to push the key into the speaking-tube which communicated with the passage. When he had pushed it as far in as his fingers could reach, he took a pencil from his pocket and pushed with that. Presently he heard the clink of the key falling on the passage floor outside. That tiresome illusion, his consciousness, was screaming in protest: his body, even had he wished, had no power to attend to those screams. Like the clockwork figure he had chosen to be, his stiff body, now terribly cold, walked back into the Objective Room, poured out the petrol and threw a lighted match into the pile. Not till then did his controllers allow him to suspect that death itself might not after all cure the illusion of being a soul--nay, might prove the entry into a world where that illusion raged infinite and unchecked. Escape for the soul, if not for the body, was offered him. He became able to know (and simultaneously refused the knowledge) that he had been wrong from the beginning, that souls and personal responsibility existed. He half saw: he wholly hated. The physical torture of the burning was hardly fiercer than his hatred of that. With one supreme effort he flung himself back into his illusion. In that attitude eternity overtook him as sunrise in old tales overtakes trolls and turns them into unchangeable stone.
From this blog - The Moment occuring after biological death:
Heaven is a choice, a decision, an act, an opt-in - and salvation therefore happens only through faith - that is love, trust of Jesus.
To understand this requires recalling the fate of the soul after the death of the body, and before the resurrection of Jesus - the soul was a witless, demented thing of little intelligence, little memory, little judgement, no free will... incapable of helping itself...
(This, at least, is how both the ancient Hebrews (with Sheol) and ancient Greeks (with Hades) regarded life after death - and other variants may be understood similarly. The soul after death was a damaged, incomplete, incapable thing - eternal life was merely eternal existence.)
I regard the Good Shepherd parable as providing the key to understanding salvation - which is that while the soul is always resurrected, resurrected Man cannot find his own way to Heaven.
The resurrected soul must be led to Heaven; that is, Man must choose to follow the guidance of the Good Shepherd. This following is not imposed, it is chosen.
This was made newly possible by Jesus because the resurrected soul has greater capability than the discarnate souls destined for Sheol/ Hades; the resurrected soul has sufficient capability to recognise Jesus, to know him; it has the capacity and necessity to choose whether to follow the Good Shepherd, or not.
Why would the resurrected soul follow the Good Shepherd to Heaven, except that the soul loved and trusted the Good Shepherd?
That is the need for faith.
Note: It might be asked why there needs to be a permanent and irreversible decision on salvation versus damnation. Why can't people change their minds?
There are a couple of aspects to this. First is that all decisions are irreversible - in the sense that they have permanent consequences. This is because Time is real, sequential, and linear. Every decision changes the sequence, changes the future; and Time cannot be rewound - because that is its nature. We know this; albeit are inclined to wish it away and that we might undo our mistakes of the past.
Second is that Heaven would not be possible unless Men were able to make a permanent positive commitment to it; a permanent commitment to Love, God, The Good and God's work of Creation. Therefore, salvation must, at some point before we go to Heaven, be irreversible.
The same need not logically apply to damnation - which is (broadly considered) the negative decision to reject Heaven; because in principle that decision might be reversed.
But in practice - as we know from the experiences of our mortal life - there is a strong tendency for choices in favour of evil to lead to further corruption, to further evil choices... and the tendency is for a choice for Good to become harder and harder, less and less likely.
This is a matter of 'psychology' rather than logic; but I think we can see from the example of Frost in That Hideous Strength how unlikely it would be that someone who has seen reality and then chosen damnation, would later reverse that decision.