“I am trying here to prevent anyone saying the really foolish thing that people often say about Him: I’m ready to accept Jesus as a great moral teacher, but I don’t accept his claim to be God. That is the one thing we must not say. A man who was merely a man and said the sort of things Jesus said would not be a great moral teacher. He would either be a lunatic — on the level with the man who says he is a poached egg — or else he would be the Devil of Hell. You must make your choice. Either this man was, and is, the Son of God, or else a madman or something worse. You can shut him up for a fool, you can spit at him and kill him as a demon or you can fall at his feet and call him Lord and God, but let us not come with any patronising nonsense about his being a great human teacher. He has not left that open to us. He did not intend to. Now it seems to me obvious that He was neither a lunatic nor a fiend: and consequently, however strange or terrifying or unlikely it may seem, I have to accept the view that He was and is God.”
So, I’m someone who thinks that Jesus was a real, historical person if for no other reason than that he was connected to other, real historical people in clumsy ways (like his connection to John the Baptist, being baptized by him).
In addition, the New Testament books go out of their way to insert arguments going on in the time they were written in order to settle them. One of those is that Jesus’s disciples just stole his body out of the tomb and lied about the resurrection. In the earliest gospel, attributed to Mark, there’s just the mystery of his disappearance, but by the time of the gospel attributed to Matthew, they have to explain why there’s this rumor the disciples took the body in addition to going into more detail about Jesus after he came back to life.
That sort of messiness, the almost complete lack of divinity in Mark and lack of all of those supposed parallels to Osiris and the virgin birth and whatnot, make me think that it’s reasonable Jesus of Nazareth was a real dude with a real life somewhat related to how it’s described in Mark, with real teachings as expressed in the gospels of Matthew and Luke, especially their sermons on the Mount/Plain.
Because we have so much evidence, both directly in the fragments of gospels we’ve found and the textual comparisons we can make, about the development of Nicene Christianity in the works that have become canon, in contrast to the suppressed works (gospels, apocalypses, pseudepigraphical epistles) deemed heretical, it seems reasonable that Jesus of Nazareth as a literary character continued to gain powers and have statements attributed to him in a way not dissimilar from how other historical and fictional characters continually accrue ever more exaggerated powers and characteristics to make additions more memorable and striking. Those accrued powers and parallels can be obviously false without making much of an impact on the historical figure.
So all of that is to say, it’s entirely consistent to be the sort of person who thinks the teachings of Jesus in Luke’s gospel are the closest to his actual, historical preaching (i.e. that ‘blessed are the poor’ is a more accurate preservation of his teachings than ‘blessed are the poor in spirit’), that everything about Matthew’s and Luke’s genealogies and maneuverings to have Jesus born in Bethlehem are superfluous to his teachings and independently fabricated to retcon him to fit a particular idea of what the Messiah would be, and that Jesus as the Son of God/God Himself are even later revisions reflecting the Gentile-centric version of Christianity as expressed by Paul of Tarsus, and as demonstrated by John’s gospel being so different from all of the rest and ever-more explicit about how Jesus’s nature is so much more than another iterant preacher and Jewish reformer.
That Jesus of Nazareth was a real person—having an understanding of Judaism that included a God much more concerned with how you demonstrate piety to the divine through specific, radical acts of love toward your fellow human beings, with greatest concern and appreciation for those who have the least, rather than assuming those who have the most possessions have been most blessed—is something you can appreciate as laudable moral teaching and radical in its emphasis on positive action rather than a negative avoiding of harm.
So to summarize, C.S. Lewis assumes the gospels are reliable, historical documents in their entirety, which is not an assumption anyone makes independent of a faith in guiding hand of the divine. If you accept that the divinity of Christ was something believers of one strain of Christianity ascribed to him after his death and was not central to his original, historical teachings, it’s entirely consistent to call him a good moral teacher by secular standards that grade on a curve for the past and while rejecting most of the rest.