Lewis saw himself as a layman in the Church of England and a Christian Apologist, not a Theologian. He was not Reform nor could he be viewed as an Evangelical; he was not an inerrantist; was an inclusivist; believed that one could reason oneself into Christianity; and opposed the penal theory of substitutionary atonement.
Mere Christianity
There are people in other religions who are being led by God’s secret influence to concentrate on those parts of their religion which are in agreement with Christianity, and who thus belong to Christ without knowing it.
https://pulpitandpen.org/2016/04/11/concerns-about-the-ministry-of-c-s-lewis/
https://sharperiron.org/article/mixed-blessing-of-c-s-lewis-part-2
D. Martyn Lloyd-Jones on Lewis
https://recognizingchrist.com/2015/01/14/martyn-lloyd-jones-on-c-s-lewis/
John Piper on Lewis
https://www.desiringgod.org/messages/lessons-from-an-inconsolable-soul
Theodicy (from the Greek words for “deity” and “justice”) refers to the attempt to justify the goodness of God in the face of the manifold evil present in the world.
To Lewis, evil and free will are related. He believed that God allows humans to be free agents with free choices. Men cannot desire freedom to choose and yet hold God responsible for not preventing the choosing of evil. Either we have freedom or we do not. Either we choose or we do not. We cannot blame God for our evil actions when we freely chose them. We cannot excuse ourselves and accuse God when freedom was truly granted to us.
Understanding what it means for God to be all-powerful must be viewed within this position.
“Try to exclude the possibility of suffering which the order of nature and the existence of free-will involve, and you find that you have excluded life itself.”
Mere Christianity
If a thing is free to be good it is also free to be bad. And free will is what has made evil possible. Why, then, did God give them free will? Because free will, though it makes evil possible, is also the only thing that makes possible any love or goodness or joy worth having. A world of automata--of creatures that worked like machines--would hardly be worth creating.
The happiness which God designs for His higher creatures is the happiness of being freely, voluntarily united to Him and to each other in an ecstasy of love and delight compared with which the most rapturous love between a man and a woman on this earth is mere milk and water. And for that they must be free.
https://www.cslewisinstitute.org/resources/c-s-lewis-on-grief/
https://www.cslewisinstitute.org/resources/c-s-lewis-on-the-problem-of-pain/
The Problem Of Pain (1940)
https://archive.org/details/in.ernet.dli.2015.264598
When pain is to be born, a little courage helps more than much knowledge, a little human sympathy more than much courage, and the least tincture of the love of God more than all.
“The Fall of Man”
The Problem Of Pain : C.s. Lewis : Free Download, Borrow, and Streaming : Internet Archive
According to the doctrine (of the Fall) man is now a horror to God and to himself and a creature ill-adapted to the universe not because God made him so but because he has made himself so by the abuse of his free will.
(The doctrine of the Fall) exists to guard against two sub-Chirstian theories of the origin of evil – Monism, according to which God Himself produces impartially the effects of good and evil; and Dualism, according to which God produces good, while some equal and independent Power produces evil.
Against both these views Christianity asserts that God is good; that He made all things good and for the sake of their goodness; that one of the good things that He made, namely the Free Will of rational creatures, by its very nature included the possibility of evil; and that creatures, availing themselves of this possibility, have become evil.
The world is a dance in which good, descending from God, is disturbed by evil arising from the creatures, and the resulting conflict is resolved by God’s own assumption of the suffering nature which evil produces.
“Human Pain”
The Problem Of Pain : C.s. Lewis : Free Download, Borrow, and Streaming : Internet Archive
Pain insists upon being attended to. God whispers to us in our pleasures, speaks in our conscience, but shouts in our pains: it is his megaphone to rouse a deaf world.
Part II
Suffering is not good in itself. What is good in any painful experience is, for the sufferer, his submission to the will of God, and, for the spectators, the compassion aroused and the acts of mercy to which it leads. In the fallen and partially redeemed universe we may distinguish (1) the simple good descending from God, (2) the simple evil produced by rebellious creatures, and (3) the exploitation of that evil by God for His redemptive purpose, which produces (4) the complex good to which accepted suffering and repented sin contribute.
“Hell”
The answer to all those who object to the doctrine of hell, is itself a question: “What are you asking God to do?” To wipe out sins and, at all costs, give (the sinner) a fresh start, offering every miraculous help? But He has done so, on Calvary. To forgive them? They will not (repent and) be forgiven. To leave them alone? Alas, I am afraid that is what He does.
A Grief Observed (1961) was Lewis’ personal journal in response to the death of his wife from cancer in 1960. He was pouring out his pain, and it is a difficult read.
https://www.snyderfuneralhomes.com/Content/Media/SnyderFuneralHome/C-S-Lewis-A-Grief-Observed.pdf
We were promised sufferings. They were part of the program. We were even told, 'Blessed are they that mourn,' and I accept it. I've got nothing that I hadn't bargained for. Of course it is different when the thing happens to oneself, not to others, and in reality, not imagination.
God has not been trying an experiment on my faith or love in order to find out their quality. He knew it already. It was I who didn't. He always knew that my temple was a house of cards. His only way of making me realize the fact was to knock it down.
Talk to me about the truth of religion and I'll listen gladly. Talk to me about the duty of religion and I'll listen submissively. But don't come talking to me about the consolations of religion or I shall suspect that you don't understand.
When I lay these questions before God, I get no answer. But rather, a special kind of ‘no answer.’ It is not the locked door. It is more like a silent, certainly not uncompassionate, gaze. As though He shook His head not in refusal but waiving the question. Like, ‘Peace, child; you don’t understand’…
God In The Dock, 1. "Evil and God"
http://www.orcuttchristian.org/Lewis%20CS%20-%20God_in_the_Dock.pdf