The fact that they believe they are C.S. Lewis makes this a really unique situation. Since I am familiar with C.S. Lewis I would ask the person which work of his they like best. I would then ask if there was anything in particular from C.S. Lewis that shows that he believed in reincarnation (the answer would obviously be no). I would then share how C.S. Lewis was a devout Christian most notable for his liar, lunatic, or Lord argument. I would ask if they, like C.S. Lewis, believe Jesus is Lord. If they did I would share with them how the two are mutually exclusive. You cannot believe in both reincarnation and resurrection.

I watched Trent Horn’s short video about how Catholics would refute reincarnation, which was helpful. I would craft my argument with one of his very logical points in mind:

“Figuring out life and who we are and where we fit in can be very challenging. I’ve found that my Catholic faith really helps give me some very reasonable answers to big questions like, is there reincarnation, or do we just have one soul and live once? An early Catholic thinker, Tertullian, came up with a very logical argument which makes a lot of sense to me. He says that if the living come from the dead, just as the dead proceed from the living, then there must always remain an unchanged amount of souls. We know that there has been a gradual growth of population over the years. So if our population is always growing, there needs to be new souls coming into existence. There aren’t enough “dead souls” to fill the new babies being born! I know you feel like you’re C.S. Lewis, but feelings often change and can be deceiving. I feel many different ways from one day to the next. But if we rely on truth and logic to answer this question, I think we’ll see that reincarnation just doesn’t make sense.”

Reincarnated, huh? I’ve never met someone who believes in reincarnation. Can you tell me more? [After she speaks for a while I would continue:] I once looked up reincarnation in the Bible to see what it has to say, because I believe the Bible always tells the truth. Here’s what the Bible says, “it is appointed for man to die once, and after that comes judgment.” Heb. 9:27 And the Book of Job says, “For when a few years have come I shall go the way from which I shall not return.” Job 16:22 What do you think about that?

Then I would wait for her answer. Depending on what she says, I might try to give her reasons to think the Bible is reliable, or I might discuss how to know which interpretation is true, or anything really. It’s hard to tell in advance, but I usually try to bring these discussions back to the Bible and then go from there depending on whether or not the person believes the Bible is true.