How was Biblical Greek pronounced?

What they all said. In the modern-day context it doesn’t matter all that much; in terms of historical reconstruction, you’re trying to pin down jelly, since the pronunciation was in flux during the period, though it seems to have been closer to Modern than Attic (though far from identical).

The reconstructions in Greek: A History of the Language and its Speakers, 2nd Edition, I think, allowed that there were different pronunciations depending on social stratum.

One thing a friend pointed out to me (hi Fiona, following us via Facebook): Anglo Christians  pronouncing Koine seem to forget it’s a human language, and put lots of awkward stops between syllables (exaggerated hiatus). Like ParOuSi. A and Agatho. Poi. E. O. I guess that works for them; but Koine was not spoken by robots, just like Homeric Greek wasn’t spoken by yodelling Martians.

See also:

• Is the modern pronunciation of Greek accurate for koine?
• What are the pros and cons of the Erasmian pronunciation?