You can't prove the Biblical god exists if your basis is the Bible being false.
Well the Bible isn't entirely true, from things we know as actual measurable facts. And given its authority is that it comes from an infallible narrator, I'd say it's a long way down from "reliable".
Bible was written by, what over a dozen different people? As far as I know, that's not where it's authority comes from. I mean, to the people who say 'bible is infallible and must be interpreted literally', maybe, but a source being unreliable, and a source being
wrong, or even just inaccurate, are separate events. A reliable source can be wrong, and an unreliable source can be right. It's just the probability. And if someone has other reasons to believe it, like, say, two thousands years of history with even now probably around half the planet believing Jesus of Nazareth was a person who existed, contentious status as Son of God and True Final Prophet No For Real This Time Guys not withstanding...
And don't get me wrong. I'm not making the 'billions of people can't be wrong' argument. Well, not in that precise way, rather. Scientific consensus sometimes ends up being wrong. Noticeable theological consensus can be hard to come by, partially because the ones they do reach a consensus just kinda disseminates into culture and thought patterns in a similar way to science (the philosophical idea of morally corrupt versus morally honorable actions, or the idea of the mind being separate in some ways from the body; and yeah there are always fringe groups who challenge these but you get the point).
But nonetheless, there's a sort of pseudo-spiritual consensus that people look at when the Abrahamic religions were so massive. Jesus was a real person, as far as I know. Miracles or not,
someone started the damned thing. And while tradition is fairly sufficient to explain it's survival past Constantine or so, it still had to get off the ground. And while it's certainly plausible Jesus was a really convincing dude with a couple of delusions, or the most successful con man in the history of the Earth, a lot of people find that to be an answer that fares worse under Occam's Razor than 'well maybe he was just telling the truth'.
So 'hey some things were lost in translation' or 'there were some errors down the years before the age of printing press' or 'the earliest existing copy of the bible is from 400 AD so who knows what got screwed up'(I may have the details wrong on that one) are all valid viewpoints. But they don't prove it false. At best you prove it to be unreliable, so people can't, well, rely on it for all their ideas. It's not an argument against theism, it's an argument against this very specific branch of theism as compared to all other branches plus non-theistic viewpoints.
Or, tl;dr and maybe just more understandable way of putting it...
Don't make symmetrical arguments unless you have proof and they don't. The field of science is in a tad bit of trouble recently, which makes it difficult to use, as there have been peer reviewed studies that 'proved' psychic powers exist, and peer reviewed studies that 'disproved' their existence. Not invalid, mind you, I'm just pointing out that expecting it to automatically support you overwhelmingly above them is....unlikely, unless it's the simple stuff like dinosaurs and age of the earth and whatnot.
"It could have happened just as coincidence, without God, so it doesn't count" is on the same level as "it could have been God, without chemotherapy, so it doesn't count". I believe one of the two is more accurate, but nonetheless symmetrical arguments are fairly useless without backup. Argue from a position of weakness instead; in this way it becomes a position of strength. Assume it's true, point out things this should lead to, allow them to correct misinterpretations....dialogue. Converse. Don't rhetoricize. And while I don't believe anyone here does it, I'll say it anyway because a. I'm a hypocrite, b. This should
definitiely keep getting longer, and c. I hear what amounts to it all the time and I'm sick of it.
"They use it too" is not an excuse for poor debate behavior. Ever.