It normally starts out with the following atheistic argument:
"Do you have proof?"
"Blah blah blah blah blah blah tl;dr: It's true because my holy text says so."
Or, for the desperate but ever so funny argument, they attempt to use religious science to prove themselves... Which will always fail and fall back to the "It's true because my holy text says so."
"Scientists say that there was a worldwide flood thousands of years ago."
"How does this prove anything?"
"It's proves so because my holy text claims there was a flood."
Most argument go back to that. "My holy text says so." It's a pathetic argument. Normally, it's countered by this:
"Using what you're trying to prove to prove itself does not work."
And that's replied to be some silly, long post related to something about God or Jesus that's complete nonsense.
After that, it goes into God. This is probably how it'd turn out:
"There is no free will."
"How so?"
"God knows everything that's going to happen."
"Yes, so?"
"Then how is it free will?"
"God works in mysterious ways." This is the tl;dr version.
"But this is contradicting."
"Well, my holy text says you're wrong."

Most arguments lead to how their religious text is right, but really...
You can't prove what you're trying to prove by using what you're trying to prove to prove it (if you can understand that).
So, why they bother doing so? I don't know.