Not my problem
You know what, I'm going to take a different route with this one. I was going to say something else (Which you likely saw) but I'm going to pivot and say this instead: If Forrest Gump is "real" in the same way that the bible is "real" then there is is no REAL moral significance to the moral presuppositions. Additionally, if the book needs to be interpret by someone who doesn't believe in the underlying axioms for people draw accurate conclusions... Then the book itself is not an accurate lens through with to view the world. His entire case for the bible is literally just "What? The glasses your wearing have ink all over them? Don't worry. I'll just tell what's happening." And then he reaches into you wallet and takes your money and then he guides you straight into a wall and walks away.
Hym "Not my problem he says. You just want the moral authority of God for yourself and if you amswer the question honestly you lose it immediately. So you obfuscate the nature of 'truth' and 'real' to appease your own conscience."
By Hym Iam