I understand that the Greek of the NT testament was Koine Greek. What can
you tell me about this?

I am not an expert on this subject, and would encourage you to do your own
research into the style of Greek used by the writers of the New Testament.
If you have a good commentary on any one of the books of the New
Testament, the author(s) will discuss in detail the style of writing of
that individual book in the introductory section.

Despite confessing that I am not an expert, let me give at least a
beginning of an answer. The study of Koine Greek was a bit of a fad in the
70’s and 80’s. Koine was used to describe the vernacular Greek of the day
in the Roman Empire. The use of Koine Greek by the writers of the New
Testament was invoked to explain some of the stylistic differences between
the writings one finds in the gospels and letters and the Greek generally
found in scholarly writings of the same time period. Probably the
significance and even the exact meaning of Koine Greek was a bit
overblown. It was claimed that Koine Greek had its own unique set of
vocabulary, some of which is found only in the New Testament, but in no
other Greek writings of the time. With time, the distinction between the
vernacular Koine and other styles has become less clear. The fact is that
the various writers of the New Testament wrote in a rather broad range of
styles, from the fairly eloquent Greek of the well-educated Paul, to the
simple and spare Greek of Mark, who clearly was not very confident in the
language. It is hard to say where koine starts and where the Greek of the
more educated classes ends, never mind the literary, scholarly Greek used
by some writers at the time. It is not as if Koine was the authentic Greek
and some sort of other style was not not authentic. By definition, any
type of Greek which was spoken or written in the first century is
authentic. However when scholars help us to make distinctions between the
different writing styles used in order to better understand what we read
in the New Testament it can be very helpful.

This is a very interesting subject which may help the careful student of
the New Testament come to a better understanding of some of the subtleties
in the New Testament writings, and I would encourage you to keep it in
mind as you do your own studies.

John Oakes, PhD