Coffman Commentaries on the Bible - Mark 15:39
And when the centurion, who stood by over against him, saw that he so gave up the ghost, he said, Truly this man was the Son of God.The centurion ... This was the officer in charge of the crucifixion. An early legend, or tradition, supported by Chrysostom, has it that this centurion was Longthus, "who was led by the miracles which accompanied the death of Christ" to become a Christian, later suffering martyrdom.[17] Whether the tradition is true or not, it is clear that the exclamation of this... read more
E.W. Bullinger's Companion Bible Notes - Mark 15:39
which = who. the Son of God = a Son of God: i.e. a supernatural or Divine being. App-98 . Found frequently in the Fayyum Papyri as a title of the Emperor Augustus, in Latin as well as Greek inscriptions. read more