And it was because Greek was the language. It is but the beginning of the re-separation of the Good Shepherd's Original Flock from that Other Flock He had gathered with it.The term "Christian" is not "commonly" used as you say, but is used twice in the singular (in Acts 26v28 by King Agrippa and in 1Peter 4v16 by Peter), and once in the plural in Acts 11v26 where it simply says "And the disciples were called Christians first in Antioch."
One may say it was the beginning of the great apostasy of men.