Unfortunately, there is no evidence that the centurion’s saying, in any form, is authentic. Versions of the saying appear only in Mark 15:39, Matthew 27:54 and Luke 23:47; but we know that both Matthew and Luke knew Mark’s gospel and borrowed from it. So there is no proof that the saying is earlier than Mark. Since the saying reflects Christian belief, it is understandable why Mark might invent a scene in which a gentile (and a Roman officer to boot!) proclaims Jesus the son of God. Furthermore, Mark puts proclamations of Jesus’ divine sonship at the beginning and at the exact midpoint of his narrative (the stories of the baptism [Mark 1:11] and the transfiguration [Mark 9:7]). So it fits his literary design to put one here as well, at the precise moment of Jesus’ death. Real history is seldom so tidy. In short, there are strong reasons to think that Mark 15:39 is a literary fiction created by Mark.