How would you respond?

[PuellaSumDei]

"How do you know people didn't just make up the Bible?"

"Well, for starters, there's ruins all over the place and documents that the Bible mentions that were also found by archaeologists. "

"But how do you know whoever wrote about it didn't find the ruins and then write up stories to explain how they were destroyed?"

:B  

[rosadria]

I'd point out that the Bible has far, far better dates as far as when the documents were written in conjunction with the event taking place than pretty much any other historical document in existence.

For example, we can date biblical texts to within a few decades of when Jesus was physically active on Earth.
However, there are only a very small amount of the book that documents Caesar's conquest of Gaul, and they are dated to centuries after the fact. They are so willing to believe that Caesar conquered Gaul, but refuse to believe that more reliable texts point to the Christ and his works on Earth.  

[Servant Reborn]

I know because I have Faith. The rest does not matter.  

[Rednal]

I'd respond in a way that's neither condescending nor assuming the correctness of my position.

You might "win" a debate, but in the end, isn't the point to convince others that you're right? Get them to agree to things step by step, and don't be afraid to admit if they have a point. Goodness knows that the Bible is full of non-literal stories (Jesus Himself frequently taught in this form...), and people who feel like they're actually being listened to are much more likely to listen to you in turn.  

[captain gemi]

They might even say they have facts like the dead sea scrolls or the Constantinople meeting. They even might say that it was written by mans hand and therefore the Bible was made up or not complete. I would say that my beliefs and faith tells me that the Bible isn't just another book. That is why I am a believer in the lord almighty.