The argument is borrowed from fellow user Ventus, and I think he summarizes it rather well:
Throughout my exchange with Ron, I fleshed this idea out into a formal logical chain, with premises labeled P and conclusions labeled C:
If there is a God, and particularly if God has revealed himself through Scripture, then ... Scripture should be better than it is.
P1. If He exists, God, by His very nature, should be perfect.
P2. Every aspect of a perfect being is, by definition, perfect.
P3. Perfect things cannot be improved.
P4. The Bible can be improved. (This claim is justified below)
P5. One aspect of the God described in the Bible is that the Bible is His word made manifest.
C1. From P1 and P2: God's Word, if it exists, should be perfect.
C2. From C1: If the Bible is God's Word made manifest, it should be perfect.
C3. From P3 and C2: If the Bible is God's Word made manifest, it should not be improvable.
C4. From C3: If the Bible is improvable, it is not God's Word made manifest.
C5. Prom P4 and C4: The Bible is not God's Word made manifest.
C6. From P5 and C5: The God described in the Bible does not exist.
In plain English, if God chose the Bible as a means of sending His truth to us, as the Bible says He did, it should be perfect or nearly perfect (there might be small errors arising from the translation from Hebrew). Certainly no human should be able to significantly improve the Bible. If I can significantly improve the Bible, then, it must not be entirely, literally right about God.
So how would I improve the Bible? I borrow two examples from Ventus. I can think of a few more, but these strike me as the most difficult to argue against, which is why I'm using them.
1. Condemn slavery. The Bible explicitly condones slavery in, for instance, Leviticus 25:44 :
"Both thy bondmen, and thy bondmaids, which thou shalt have, of the heathen that are round about you; of them shall ye buy bondmen and bondmaids."
That's from the King James version; Merriam-Webster defines "bondmen/maids" as people bound to service without wages (ie. slaves), and newer translations just use the term "slaves" explicitly. In other words, it's ok to have slaves as long as they're heathens. Support of slavery can also be found in the New Testament (eg. Ephesians 6:5).
Even without that explicit condoning, though, what does it mean to not have an anti-slavery passage in the Bible? I realize that the Bible can't be expected to condemn every immoral act by name, but Leviticus makes a good effort to do so; it's mostly just a big list of God's rules for mankind. Chapter 11 is all about what you are and aren't allowed to eat, with several helpful examples. Chapter 18 is just a big index of S@xual taboos, and chapter 20 is another index of the same taboos but with the death penalty attached to most of them. In all of this laying-down-the-law, how is it that God neglected to condemn the appalling human rights violation that is slavery, but remembered to note that we can't eat rabbits because they "chew the cud"?
2. Get rid of Genesis 19:8, or at least modify it heavily. The one argument I've seen against this is that Lot offering his daughters to the rapists was an act of man, not an act of God, so God shouldn't be judged for it. But Genesis 19 is not just a record of what happened one day in Sodom way back when. It's meant to teach a lesson; the Sodomites are punished because they've strayed too far from the righteous path, Lot is saved because he puts his trust in God, and Lot's wife is punished for even looking back to the cesspool of sin from which she was fleeing. Lot is clearly the protagonist in this story; his hospitality saves the travelers from the mob, and his faith saves him when the rest of the city burns. Why is it never discussed that by offering his daughters to the rapists, he is committing the second most immoral act in the chapter (right after the rapists themselves)? If the angels who saved him were really messengers of a good God, they would have at some point explained to him that his actions were despicable. And since Genesis 19 is a story about morality, this explanation should have made its way into the final draft.
What do people think? This reasoning can be debunked by either pointing out a flaw in my logic or disagreeing with one of my five premises; I expect that most people will disagree with P4, which I why I spent so much time justifying it, but I'm happy to debate any of the other premises if someone sees them as mistaken.
Finally, a couple of notes, to avoid messy issues raised in the other thread:
-I realize that this evidence doesn't apply to belief in all gods, or even in most gods. It only applies to belief in the God of the literal Bible.
-Yes, my user profile says I'm a Christian, and yet I'm saying the Bible can't be literally true. If you want to discuss that, start a thread or direct me to a thread that already exists; it's not relevant to the topic of this thread (on the off chance that you think it is, please explain why in detail).
-The "historical evidence for Jesus" argument should be nipped in the bud before it starts. My argument is based entirely on logic. It's certainly debatable, but historical evidence can't beat it; documents and testimonies which suggest that the Bible is true, however convincing they are, are trumped by a deductive logical proof that the Bible cannot be true. (Again, if you think I'm wrong here, please explain thoroughly before you present all your evidence for the historicity of Jesus.)
-The claim that "I'm just writing opinions, not giving evidence" is objectively false. Logical proof is a form of evidence. If you think that one of my premises is just an unfounded opinion, tell me which premise you have a problem with and why.