One of my recent personal projects is to learn to read koine Greek, or, more specifically, New Testament Greek, so called because it is the form of ancient Greek in which the New Testament was originally written. My ultimate goal is be able to read at least part of the New Testament in its original language, but given the rate at which I am learning the language--I've only been able to study it for approximately half an hour per week-- I will probably not reach that goal in years! The text I have been using to learn New Testament Greek, Complete New Testament Greek, was authored by Gavin Betts, a former associate professor of classical studies at Monash University in Australia. In the book, Prof. Betts notes that "when we compare New Testament Greek with the form of [Greek] spoken in other parts of the contemporary [i.e., the first century AD] Greek world or with the classical language (i.e., Attic of the fifth and fourth centuries BC) we find that it contains Semitic elements of Hebrew or Aramaic origin" (p. 7). I should perhaps point out that among the Jews living in what we call the Holy Land in the first century AD, Aramaic was more commonly used as an everyday language than was Hebrew. However, both are Semitic languages.
Prof. Betts goes on to say that these "Semitic elements of Hebrew or Aramaic origin" that appear in New Testament Greek include points of style, Semitic idioms, and transliterations of Semitic names into the Greek alphabet. An example of the first category would be "the Hebrew practice of joining clauses by and rather than by subordinating one to another, as was normal in Greek" (p. 7). An example of the second type would be an expression like "son of peace," meaning "a peaceful person." Using the word "son" metaphorically to mean someone associated with some quality was common in Semitic languages but would have been "quite unidiomatic by the standards of classical Greek." An example of the third type (transliteration) would Iohannes for the name John.
The Semitic idioms that appear in New Testament Greek (the second type of "Semitic elements of Hebrew or Aramaic origin") are often called Semiticisms. The existence of Semiticisms reflects a rather common phenomenon in language. When someone is using a language other than his or her own native language, it is very easy to fall into the habit of using the non-native language in a way that is typical of one's native language. As a teacher of English as a second language (ESL), I am very familiar with this phenomenon. For example, very often my Chinese-speaking students will speak or write "Chinglish"--English words used to literally translate Chinese expressions. Sometimes it takes me a while to understand what the students are actually trying to say, but with a little reflection on my own knowledge of Chinese or some help from my students, I am usually able to figure out what they are attempting to communicate. This is, of course, not a phenomenon exclusive to Chinese speakers learning English, as witness the existence of such forms of English as "Spanglish" or the various types of pidgin English.
In short, since the writers of the New Testament (with perhaps the exception of Luke, the author of the Gospel of Luke and Acts of the Apostles) did not speak Greek as a first language, it should not come as a surprise that they sometimes wrote Greek as if they were writing in their native language. Nevertheless, by choosing to write in Greek--even if not always perfectly idiomatic Greek--they were able to make the "Good News" of the Christian faith known more quickly than if they had chosen to write in their native language.
Image of ancient Greek alphabet from dkfindout.com