See ARAMAIC LANGUAGE for proof that Jesus spoke that language as the vernacular of the people of Palestine. But Christ spoke the koine also, so that the New Testament is not an idiom that was unknown to the Master. Gwilliam (1-vol HDB, "Language of Christ") does still deny that Jesus spoke Greek, while Roberts takes the other extreme in his book, Greek the Language of Christ and His Apostles (1888). Per contra again, Julicher considers it impossible to suppose that Jesus used Greek (art. "Hellenism" in EB). J. E. H. Thomson, "The Language of Palestine during the Time of our Lord" (Temple, Bible Dictionary) argues convincingly that Palestine was bi-lingual and that Jesus knew and spoke Greek as well as Aramaic Peter evidently spoke in Greek on the Day of Pentecost and was understood by all. Paul was understood in Jerusalem when he spoke in Greek (Ac 21:37). Jesus taught in Decapolis, a Greek region, in the region of Tyre and Sidon (Greek again). Galilee itself was largely inhabited by Gentiles who spoke Greek. At the time of the Sermon on the Mount, we read that people were present from Decapolis and Perea, besides the mixed multitude from Galilee, Judea, and Jerusalem (Mt 4:25; Lu 6:17). Thomson proves also that in Matthew's Gospel the quotation from the Old Testament in the words of Jesus is from the Septuagint, while Matthew's own quotations are from the Hebrew. The case seems clear. It is not possible to say always when Jesus spoke Greek and when Aramaic. That would depend on the audience. But it is practically certain that Christ Himself knew and spoke at will the vernacular koine, and thus had this linguistic bond with the great world of that era and with lovers of the Greek Testament today.
LITERATURE.
The literature on this subject is very extensive. The most important volumes have been mentioned in the discussion above.
A.T. Robertson