Since hearing John Fenton expound this reading, I have often taught it myself. Usually people are somewhat shocked at first, but as time goes on the reading becomes more appealing and, ultimately, quite persuasive. If the cry is sarcastic, it makes sense in its narrative context. Take another look:
Mark 15.37: ὁ δὲ Ἰησοῦς ἀφεὶς φωνὴν μεγάλην ἐξέπνευσεν. 38. Καὶ τὸ καταπέτασμα τοῦ ναοῦ ἐσχίσθη εἰς δύο ἀπ' ἄνωθεν ἕως κάτω. 39 Ἰδὼν δὲ ὁ κεντυρίων ὁ παρεστηκὼς ἐξ ἐναντίας αὐτοῦ ὅτι οὕτως ἐξέπνευσεν εἶπεν, Ἀληθῶς οὗτος ὁ ἄνθρωπος υἱὸς θεοῦ ἦν.There is nothing that the centurion has seen that suggests that we should read the text as implying his admiration for Jesus. He makes his comment when he saw how Jesus died (ὅτι οὕτως ἐξέπνευσεν εἶπεν), that is, in despair (15.34), apparently unable to call down Elijah to deliver him (15.35-6). The reader, on the other hand, is given some privileged information, that the veil of the temple was torn in two, from top to bottom. This is classic dramatic irony: the reader knows what the actors in the drama do not know, and the actors do not realize the truth of the words that they are speaking in mockery.
Mark 15.37: And Jesus gave a loud cry and breathed his last. 38. And the veil of the temple was torn in two, from top to bottom. And when the centurion, who was standing facing him, saw how he died, he said, "Truly this was a son of god".
To read the text in this way coheres with the rest of Mark's Passion Narrative, which is commonly regarded as rich in irony. Jesus is repeatedly mocked as a king (15.9, 12, 18, 26, 32) with purple cloak, crown of thorns and mock homage (15.17-20), but the reader knows that he really is a king. He is mocked as a prophet (14.65) while his very prophecies are being enacted all around him (the mockery itself, fulfilling his Passion predictions, and Peter's denial, fulfilling Jesus' Last Supper prophecies). Given this context, it is difficult to think that the centurion's remark can be intended as a "confession" of faith in Jesus. Reading the remark as the crowning element in the dramatic irony of Mark's Passion Narrative makes good narrative sense.
Now, the first time I heard the suggestion that we read the centurion's cry as ironic was from my tutor John Fenton. A friend tells me that he thinks Fenton got this interpretation from Austin Farrer. This is plausible because Fenton was an admirer of Farrer, but I don't remember that from what Fenton himself said -- and he was inclined to attribute ideas that were not original. I have cast around the literature too to see if anyone else has read the text in this way, and I am surprised to be struggling to find examples of this interpretation. I had thought that perhaps Donald Juel mentioned it, but again I can't find it. Fenton himself mentions it in written work once, as far as I am aware, as follows, with some helpful additional context connecting the saying to the Elijah on Horeb narrative:
Elijah had, it was believed, set up an experiment to prove that the Lord was God, not Baal (1 Kings 18). Someone now tries to repeat the experiment on Jesus: he is given the drink, and they say, Let us see if Elijah will come to take him down from the cross. If Elijah comes, Jesus is who he says he is; if Elijah does not come, he is not. Jesus dies, without the intervention of Elijah, thus proving to those who think in this way that he was not the Messiah, the king of Israel. (It is possible that the centurion's words should be taken in this sense: He really was God's son! Of course not! There is a parallel in the Greek between what the people said on Mount Horeb after Elijah's miracle: Truly the Lord is God; and what is said here, Truly this man was the Son of God. There is also the possibility that 'this man' should be translated 'this fellow', disparagingly, as in Acts 6.13.), John Fenton, Finding the Way Through Mark (London: Mowbray, 1995), 111.If anyone has any more from the literature on this theme, I would be interested to hear it.