Earl S. Johnson, «Mark 15,39 and the So-Called Confession of the Roman Centurion», Vol. 81 (2000) 406-413
Continuing examination of the grammatical, literary and historical evidence indicates that the centurion's remarks about Jesus in Mark 15,39 cannot be understood as a full Christian confession of Jesus' divine sonship, and cannot be taken as a direct challenge to any Roman emperor in particular. Jesus' identity in the gospel is not revealed by the centurion, the demons, the disciples or in the introduction to the gospel. It is made clear by God's declaration that he truly is the Son (1,11; 9,7), and in the faith of the readers as they search for Jesus' presence in their own community.
that understanding it differently there will undermine that keystone of our faith.
To say that the centurion serves as a symbol for the incipient promise of Jesus in 13,10 that the gospel will be preached to all the nations31 is to make of him a kind of deus ex machina or a rabbit out of a hat, who conveniently appears out of nowhere, ties up all of our questions and all of Marks loose ends. But the gospel does not allow itself to be interpreted or outlined in such a manner. Marks original text is the one which has no reference to the Son in 1,1; he provides the anarthrous ui(o_j qeou=; he is the one, as far as we know, who has written an unusual ending, completing his gospel with an enigmatic ga/r, pushing readers into the community of faith to find the answers to the riddles he poses. If that is where Mark goes, we have no choice but to follow him there.