Mark Goodacre has served us well with the reminder that Mark 15:39 need not necessarily be taken literally, but rather as another sarcastic taunt!
Is this a confession of faith, a meaningful statement or the final taunt of a mocker who stood by while Jesus was executed? Scholars seem to suggest at least one of these three.
Evans, Mark 8:27-16:20, pg. 510
Evans supposes that the centurion sees the power of Jesus death and the torn temple veil and this leads to his confession. It is further conjectured that this leads the centurion to switch allegiance from Caesar to Christ. Although it is admitted that this is not an ‘orthodox’ Christian confession, it is proposed that the centurion is impressed enough with the available details to ascribe “to Jesus what he earlier ascribed to Caesar.”
The two lines of evidence that Evans employs to justify this verdict are seen to be unreasonable. Firstly, it is unlikely that the centurion could see the torn temple veil. The distance, the crowds, his focus on the immediate situation make it highly improbable that this is the case. Secondly, what is there to suggest that the centurion would make this connection? Mark may have made the connection, but it is dubious to suggest that we know that the centurion would or did make this connection. Thirdly, there was nothing impressive about Jesus’ death. It was an utter shame and disgrace. There is nothing about the pathetic death of Jesus that commends itself as impressive. This was just another wannabe Jewish rebel who died at the hands of a ruthless imperial lord.
France, The Gospel of Mark, pg. 658-660
This argument begins by noting the geographical implausibility of the centurion seeing the tearing of the veil. Even at the narrative level, France notes, this is impossible and Mark does not say that the centurion saw the curtain tear.
France marshals the circumstantial evidence to suggest what could possibly have impressed him so deeply so as to make this confession. It is noted that “his manner of death has proved the truth about what he has been in life.” It is then suggested that what matters is that Mark’s readers see this as the triumphant declaration of who Jesus is.
Edwards, The Gospel According to Mark, pg. 479-81
Appealing to theological motifs, Edwards proposes that “the fact that the passion and death of Jesus on the cross evoke the confession of the centurion indicates that he, by divine revelation, has been granted the mystery of faith in Jesus as the Son of God.” However, it seems unnecessary to impose a theological rationale at this stage of exegesis. Before jumping to theology, one must carefully consider the historical factors at work. Edwards himself notes Martin Hengel’s conclusion which notes that “a crucified messiah, son of God or God must have seemed a contradiction to anyone, Jew, Greek, Roman or barbarian, asked to believe such a claim, and it will certainly have been thought offensive and foolish.” Is there any evidence in Mark to suggest that this is a divine revelation? Does Mark’s narrative lead us to this conclusion?
Witherington, The Gospel of Mark, pg. 400
Witherington adopts the understanding that the confession is akin to the Hellenistic model of son of god. It is at least conceded that the alternative that I shall propose is ‘possible’ with a reference to a proponent of this view footnoted.
Hooker, The Gospel According to Saint Mark, pg. 378-9
“… while it is true that the centurion, if he uttered these words, could only have meant by them a divine man or demi-god, yet for Mark they are a proclamation of the truth about Jesus… Whether Mark thinks that the centurion is aware of the true significance of his words is not clear. Perhaps Mark regards them as an unconscious acknowledgement of Jesus’ identity, like the taunts of those who mocked the dying Jesus, unaware of the true meaning of their words (15:18, 26, 29f., 31f.), and the incredulous questions of the high priest and Pilate (14:6; 15:2). The truth is thus spoken by Jesus’ judges and by his executioner. Nevertheless, the centurion stands at this point as the representative of those who acknowledge Jesus as God’s son.
However, if the centurion did offer such a statement, and it was remembered by the women who, after hearing this statement left the scene, what could the centurion have meant?
- Mk 15:18 And they began saluting him, “Hail, King of the Jews!”
- Mk 15:26 The inscription of the charge against him read, “The King of the Jews.”
- Mk 15:29-30 “Aha! You who would destroy the temple and build it in three days, save yourself, and come down from the cross!”
- Mk 15:31-32 “He saved others; he cannot save himself. Let the Messiah, the King of Israel, come down from the cross now, so that we may see and believe.”
- Mk 15:36 “Wait, let us see whether Elijah will come to take him down.”
- Mk 15:39 Now when the centurion, who stood facing him, saw that in this way he breathed his last, he said, “Truly this man was God’s Son!”
Given the context of mockery and disgrace it remains more likely that the confession is the final of a series of mocks by a host of different voices. Firstly, nearly all of the disciples have abandoned him. The titulus is an imperial mock, the scribes and priests offer various taunts, and finally the one who crucified Jesus offers the final nail in the coffin by sarcastically praising Jesus with the title reserved for Caesar. The so-called confession is about as meaningful as the titulus or declaration that Jesus is the saviour by the priests.
Is it historically plausible, and exegetically viable to suggest that the centurion’s confession is somehow meaningful and an accurate representation of his allegiance? It seems unlikely, thus we follow Goodacre, Fenton and Juel who see this is a sarcastic remark akin to the other mocks that Jesus has received.
 Hengel, Crucifixion in the Ancient World and the Folly of the Message of the Cross, pg. 10  See Goodacre for the Fenton reference, and Juel, Messianic Exegesis, pg. 28, 146.