Can't a lunatic be a great moral teacher? Goodness and understanding of reality seem to be orthogonal (or even opposed as many people who are, or at least think they are, realists, are quite cynical).
For statements like, "do unto others as you would have them do unto you," sure. Lewis' point is that he made all kinds of other claims that are either true or false. And if they are false, they are false in a way that led to lots of people dying for no reason. So Jesus was either saying those things that led to all his followers being killed either because he was crazy and believed what he said, because he was a liar and evil, or because he was telling the truth and good.
By what would the lunatic ground their thinking? From one angle, I can respect your argument as in a case where someone opines "Wow, look at Crazy Pete. He sure is crazy but he's nice to animals. Maybe I should be nice to animals too, like he is!"
In the case of Jesus, he told his followers to eat him (John 6). Potentially normal lunatic stuff...when they were like "whoa wtf bro" he doubled down and kept using a word which describes teeth tearing and chewing. I don't find cannibalism to be highly moral (since it usually involves murder) so shouldn't that disqualify Jesus from being a lunatic but also a good moral teacher?
You seem to assuming that all of ones teachings must be moral in order for one to be able to be a good moral teacher. But if people are able to pick and choose teachings then this may not be required. As an example, many people would consider the bible to be a good source of moral teaching, but most would admit that it contains some teachings which are outright reprehensible.
If someone needed to be entirely good to be a good moral teacher then almost nobody (if anybody) would qualify.
In Matthew 5:48, Jesus tells us to "be perfect as your Heavenly Father is perfect". I'm just holding him to his own moral standard :)
Moral authority is only as powerful as the believers not finding out you don't ACTUALLY believe what you are telling them to. People easily write off the Catholic Church because of the disgusting abuses Catholic authority figures have perpetrated throughout the ages. Pick and choose works with bare philosophies but Jesus isn't asking us to believe right, he's asking us to live right. You can't pick and choose living right, either you are or you aren't. If you only sometimes rob the bus station you aren't living right.
Catholics go to confession for this exact reason - people still mess up both accidentally and on purpose, and when they do they need a path back to God. This is the concept of repenting, where you just turn around and walk away from your sin and back to God, like the Prodigal Son.
Living right is acknowledging the mistakes and trying to never do that same thing again. Repeatedly robbing a bus station is quite a far cry from trying not to do evil on purpose.
To follow this lunatic you have to eat his flesh. From John 6:
> 51 I am the living bread that came down from heaven. Whoever eats this bread will live forever. This bread is my flesh, which I will give for the life of the world.”
> 52 Then the Jews began to argue sharply among themselves, “How can this man give us his flesh to eat?”
> 53 Jesus said to them, “Very truly I tell you, unless you eat the flesh of the Son of Man and drink his blood, you have no life in you. 54 Whoever eats my flesh and drinks my blood has eternal life, and I will raise them up at the last day. 55 For my flesh is real food and my blood is real drink. 56 Whoever eats my flesh and drinks my blood remains in me, and I in them. 57 Just as the living Father sent me and I live because of the Father, so the one who feeds on me will live because of me. 58 This is the bread that came down from heaven. Your ancestors ate manna and died, but whoever feeds on this bread will live forever.”
Similarly at the Last Supper, where he also instructs his followers to drink his blood:
> 27 Then he took a cup, and when he had given thanks, he gave it to them, saying, “Drink from it, all of you. 28 This is my blood of the[b] covenant, which is poured out for many for the forgiveness of sins.
Some folks (even Christians, like some Protestant denominations) believe it is meant to be symbolic, but the earliest Christians did not take it symbolically:
So what is Jesus teaching when he says that you must love [Hh]im above all, and that [Hh]e is the truth itself?
> 37 “Anyone who loves their father or mother more than me is not worthy of me; anyone who loves their son or daughter more than me is not worthy of me. 38 Whoever does not take up their cross and follow me is not worthy of me. 39 Whoever finds their life will lose it, and whoever loses their life for my sake will find it.
> 6 Jesus answered, “I am the way and the truth and the life. No one comes to the Father except through me. 7 If you really know me, you will know[b] my Father as well. From now on, you do know him and have seen him.”