There are two general lines of objection to Lewis's argument. One of them is the Higher Critical objection and the other the Sincere Mistake objection.
Stephen Davis's challenge to the higher critical objection is that even if you go as far with higher criticism as the Jesus Seminar goes, (which means, for example, things like "Before Abraham was, I am (YHWH) would be excluded as products of the early church), there are passages rated at least pink (Jesus probably said something like this) like
"But if it is by the finger of God that I cast out the demons, then the kingdom of God has come to you." (Luke 11: 20)
This clearly means that Jesus actually thought he was exercising God's eschatological power in exorcisms.
OR "Listen to me all of you, and understand, there is nothing outside a person that by going in can defile, but the things that come out are what defile. (Mk 7: 14-15).
So Jesus claims the authority to say that we are not defiled by eating the wrong foods? And he's telling this to observant Jews? Who does he think he is?
or "The sabbath was made for humankind, and not humankind for the sabbath; so the Son of Man is Lord even of the sabbath." Mk 2: 27-28.,
Lord over the Sabbath?? Is he saying he made the Sabbath the Sabbath, and can set it aside if necessary? Again, who does he think he is?
These sayings are rated as likely to have actually been said by the Jesus Seminar.
It's also important to realize that the Jewish leaders reacted just as we would expect them to react to (to their mind) inappropriate divine claim. In response to Jesus' claim to forgive sins, The Jewish leaders don't say "No one can get their sins forgiven except by going to the temple," they say "No one can forgive sins but God alone." These sayings are rated as likely to have actually been said by the Jesus Seminar.
And notice the other problem. Suppose you think that a purely naturalistic Jesus has to be found. The cartoon image I have in my mind is some kind of first-century hippie guru who teaches peace and love, told good stories, and didn't claim any kind of supernatural prerogatives. It was his followers dragged in all the supernatural stuff and made a religion out of this simple leader's teaching (especially that jerk Paul). So everything is inauthentic that can't be fitted in nicely with philosophical naturalism.
But I have no idea how to peel the onion back so that we can get a historical Jesus who fits nicely with naturalism, and then throw the rest out as a product of the early church. Is there anything in the text that actually supports that kind of reading? Second, if we go that way with respect to Jesus, why would anyone bother to crucify him?
Beversluis's effort is a creditable one from the critical side. But Davis's essay from the Oxford Press anthology on the Incarnation, is an extremely important one, and although Beversluis responds Davis at one point in the essay, he doesn't deal at all with Davis's argument against the Higher Critical objection, which I have given only a small part of here.
I'm linking to the book where Davis's essay appears.