Answer:
The New Testament of the Bible was written in Greek because Greek was the common language of the Roman Empire. Even though Romans didn't speak Greeks they wrote the Testament in Greek. This was done as Greek was more widespread than the language they spoke. Greek usage ensures that their manuscripts could be widely read and passed on to future generations. It was also done to spread Christianity in a common and well understood language.
Next Question: Why is bhang consumed on 'Mahashivratri'?
Previous Question: Why is Jesus Christ, often known as 'Jesus of Nazareth'?