By the language and the internal content we have come up with a pretty good idea who the audience was for each book. Most scholars seem to agree on this. Having translated all the gospels from the Greek text, I concur that we have it down pretty well.
Matthew was written to the Jews to prove that Christ was the Son of David and of course a descendent of Adam.
Mark was written to the Romans because it is a letter short and to the point and meant to explain to them the efficiency of the Lord as a Leader. This would appeal to the Roman mind.
Luke was written in almost classical Greek by physician Luke. Check out my Luke commentary that I wrote in my commentary series. It was written mainly for the Greeks and shows that Christ was the Son of Man, or of Mankind.
John was written to the Gentiles in a wide and broad way. He explains many words and ideas that would help the Gentile under Jewish issues and geographical locations. The book is easy reading and perfect for explaining to the Gentile world the nature of the Savior.
Thanks for asking.
Dr. Mal Couch (7/12)