In what language was the New Testament written: Greek or Aramaic?
The New Testament was written in Greek. All the earliest manuscripts are in Greek. The style of writing is in the Greek style, though some of the writers, such as Matthew, wrote with a Hebrew "accent," if you would, but it was still in Greek. See Was Matthew originally written in Hebrew or Aramaic? for details.
Probably the easiest proof is to note that frequently when a statement is transliterated from Aramaic or Hebrew, the meaning is translated for the reader into Greek.
- "Behold, the virgin shall be with child, and bear a Son, and they shall call His name Immanuel," which is translated, "God with us"" (Matthew 1:23).
- "And about the ninth hour Jesus cried out with a loud voice, saying, "Eli, Eli, lama sabachthani?" that is, "My God, My God, why have You forsaken Me?"" (Matthew 27:46).
- "Then He took the child by the hand, and said to her, "Talitha, cumi," which is translated, "Little girl, I say to you, arise"" (Mark 5:41).
- "And they brought Him to the place Golgotha, which is translated, Place of a Skull" (Mark 15:22).
- "And at the ninth hour Jesus cried out with a loud voice, saying, "Eloi, Eloi, lama sabachthani?" which is translated, "My God, My God, why have You forsaken Me?"" (Mark 15:34).
- "Then Jesus turned, and seeing them following, said to them, "What do you seek?" They said to Him, "Rabbi" (which is to say, when translated, Teacher), "where are You staying?"" (John 1:38).
- "He first found his own brother Simon, and said to him, "We have found the Messiah" (which is translated, the Christ). And he brought him to Jesus. Now when Jesus looked at him, He said, "You are Simon the son of Jonah. You shall be called Cephas" (which is translated, A Stone)" (John 1:41-42).
- "And Joses, who was also named Barnabas by the apostles (which is translated Son of Encouragement), a Levite of the country of Cyprus" (Acts 9:47).
- "For this Melchizedek, king of Salem, priest of the Most High God, who met Abraham returning from the slaughter of the kings and blessed him, to whom also Abraham gave a tenth part of all, first being translated "king of righteousness," and then also king of Salem, meaning "king of peace"" (Hebrews 7:1-2).
One doesn't give translations when the work is written in that language. For detailed arguments, see the rebuttal to "The New Testament was written in Hebrew, not Greek!"