It is true, certainly, that it is a Greek New Testament which presents itself to the scholar for study, but within the New Testament there are portions of which "the original language" was not Greek but Semitic. Jesus of Nazareth, the Man whose personality was the decisive impulse, did not speak Greek when He went about His public work. He spoke the local idiom of His native Galilee, the language which, in the night of betrayal, betrayed His disciple Peter to be a Galilean. This language was Aramaic, a dialect akin to Hebrew but not identical with it; and, to be quite exact, it was Galilean Aramaic that our Lord spoke. In that dialect the gospel was first preached. The ordinary reader of the Bible even now hears the last echo of the original when he comes upon such words as mammon, talitha cumi, abba, or such names as Barabbas, Martha, etc., which are all of them Aramaic. Moreover, the oldest record of the words that Jesus spake, the record of His apostle Matthew, was no doubt written in Aramaic for the Palestinian Christians who spoke that language. That most primitive version of our Lord's words has perished, unfortunately, so far as the Aramaic original is concerned. What would we give if we could recover but one papyrus book with a few leaves containing genuine Aramaic sayings of Jesus! For those few leaves we would, I think, part smilingly with the theological output of a whole century.
But it is little use to speak further of this "if." It is more sensible to inquire why the words of Jesus are no longer extant in their original Aramaic. The answer is that Christianity, in becoming a world religion, gradually forgot its oldest records- records that had originated far away from the world and were unintelligible to the world- and so they were lost. The Christian missionaries with an Aramaic book of gospels in their hands would have been powerless to make propaganda in what was in fact a Greek or rather Hellenised world. An Aramaic gospel book would have condemned Christianity to remain a Palestinian sect. Ere it could become a world religion it had to learn the language of the world, and that is why the gospels put on the habit of the world; for that reason St. Paul and others spoke and wrote the international language, and the New Testament took final form as a Greek book.
from pages 64- 65, Light from the Ancient East