I'm an atheist but I've always appreciated CS Lewis' and GK Chesterton's writings on Christianity and Orthodoxy, and I don't think it's a bad argument in the right context. I don't think the argument from Lewis is that nobody can be a great moral teacher and not be god, but that Jesus specifically as he's described in all four gospels. If you take the Gospel of John being as legitimate as the other Gospels, no more or less true than the others, the Jesus does certainly allude to himself as being God or having some unique relationship with God which starts to resemble the Trinity. For the early Church at least, the words attributed to Jesus by John make a strong argument for the divinity of Jesus. I think Lewis' argument in Mere Christianity is if you place weight in Jesus as a moral teacher, that you think there's some foundation there and that he's not a raving madman, then you can't separate Jesus in Matthew, Mark, and Luke from Christ in the Gospel of John.
I think Lewis is best when he follows Chesterton, making an argument from Orthodoxy to other Christians, not with -- say -- atheists or humanists. The audience he was usually speaking to was a Christian-friendly secular audience, where you saw this trend of moving away from the divinity of Christ and more to the idea that Jesus is like Aristotle. I think Lewis is right that Jesus cannot be Aristotle, Jesus didn't seek his moral authority through reason, but through faith, so in the 20s and 30s when Christian-adjacent humanists were reconsidering the divinity of Christ while still preserving the teachings of Jesus, as long as you take the the four gospels as co-equally legitimate, then challenging Christ's divinity challenges faith in Christ which ultimately challenges Jesus' moral teaching.
It's not that Lewis is arguing that "one can't be a good moral teacher without being god," but that for a Christian, Jesus specifically cannot be a "good moral teacher" and not be god. If you take the words of the gospels as all legitimate, then Jesus must be mad or he must be god. The most compelling argument against this, of course, is that the four gospels shouldn't be read as co-equal and they can't be interpreted as the literal word of god, that they're a product of the communities that they were written in, and in the ensuing ~70 years after Jesus' death (or whatever the latest historicity timelines of the gospels are), that some communities came to develop different theories of Jesus and Christ based on what was important to them at that time. To me, an atheist, this seems like the most reasonable and historically accurate way to read the gospels and still find a lot of value in them, but for an Orthodox Christian, Catholic, Eastern, etc (or even probably Anglican), Lewis would argue, y'know, you're rreally not supposed to pick and choose which "word of god" is the "word of god."
When reading St. Thomas, Chesterton, and C.S. Lewis, they all sort of start from the same precept: "These things all being true... (...) ...These things must follow." So coming to them without agreeing with that first part then the second part becomes flimsy. I don't believe in god, and so I don't believe those first things are true, but if I did believe those first things are true, then I think there's a simple logic. I think Lewis was right in limiting the scope of Mere Christianity, especially for the context: He was giving the lectures to counter English-language Nazi propaganda that was over-taking British airwaves whenever there wasn't something else being broadcast. Chesterton didn't limit his scope in the same way that Lewis did early on, and which is what makes Chesterton so fun, but also much more flawed and subsequently less well known... unless you're really into Christian apologetics or early 20th century humorists/satirists.
en.wikipedia.org
