I have read C.S Lewis books, S.T.L; mere Christianity, the great Divorce et al. Love his philosophical approach in answering skeptic's, he does a masterful job.
He is not capable of avoiding to the stumbling blocks of scripture any more than I am. The philosophical reasoning of man though very useful and powerful in its own arena cannot succeed in overcoming the revelation of scripture that counters the intuitive/philosophical capabilities of men.
One cannot assume the freewill of man without explicit scripture validating the freewill of man as the necessary agent of change to bring about a change of heart or mind.
Matthew 13 New King James Version (NKJV)
The Parable of the Sower
13 On the same day Jesus went out of the house and sat by the sea. 2 And great multitudes were gathered together to Him, so that He got into a boat and sat; and the whole multitude stood on the shore.
3 Then He spoke many things to them in parables, saying: “Behold, a sower went out to sow. 4 And as he sowed, some seed fell by the wayside; and the birds came and devoured them. 5 Some fell on stony places, where they did not have much earth; and they immediately sprang up because they had no depth of earth. 6 But when the sun was up they were scorched, and because they had no root they withered away. 7 And some fell among thorns, and the thorns sprang up and choked them. 8 But others fell on good ground and yielded a crop: some a hundredfold, some sixty, some thirty. 9 He who has ears to hear, let him hear!”
The Purpose of Parables
Matt 13:10 And the disciples came and said to Him, “Why do You speak to them in parables?”
11 He answered and said to them, “Because it has been given to you to know the mysteries of the kingdom of heaven, but to them it has not been given. 12 For whoever has, to him more will be given, and he will have abundance; but whoever does not have, even what he has will be taken away from him. 13 Therefore I speak to them in parables, because seeing they do not see, and hearing they do not hear, nor do they understand. 14 And in them the prophecy of Isaiah is fulfilled, which says:
‘Hearing you will hear and shall not understand,
And seeing you will see and not perceive;
For the hearts of this people have grown dull.
Their ears are hard of hearing,
And their eyes they have closed,
Lest they should see with their eyes and hear with their ears,
Lest they should understand with their hearts and turn,
So that I should heal them.’
16 But blessed are your eyes for they see, and your ears for they hear; 17 for assuredly, I say to you that many prophets and righteous men desired to see what you see, and did not see it, and to hear what you hear, and did not hear it.
In our assumptions we build a philosophy and then ideologies, human free will is not exempt. In our attempt at making sure we carry all the aces when it comes to God's salvation, we are happy to develop philosophies that secure our own ability to check God's power and stay his hand according to our own whims.
The above explanation of the parables warns us all that sinful/hardened hearts see, hear and with that seeing and hearing develop reasoning and justifications for themselves even using the prophets and the law as did the people and the Pharisees, but seeing they didn't understand, and hearing they didn't gain knowledge from God, so it is with mans free will, it pretends to know and then own whatever it desires but Jesus warns his disciples they are deceived...for unto them it is given to know...and for others it is not given for them to know. Their freewill even in the face of Jesus and his teachings could not secure from God what was being spoken.
C.S. Lewis stumbles at God saying NO, even though we have told ourselves in myriads of ways God never says no only yes.