Great art stands alone. As we move from awe to analysis, we begin to work out how the artist achieved his effects, but such details are merely the scaffolding of genius. Technique is mastered – often revolutionised – and sublimated into the finished work. In great art, it is impossible to separate form and content. How can we tell the dancer from the dance?

Equally, and even if we mere devotees are nosy, biographical information belongs in the foothills of greatness. Obviously, the artist as human being is the product of his family and his society, but the human being as artist transcends both.

Even so, there is one aspect of religious art which intrigues us. What, if anything, did the artist believe? It would be difficult to accept that the composer of the St. Matthew Passion believed in nothing, or that the composer of the Missa Solemnis believed in much. If we consider the Renaissance, when no artist could make a decent living without religious commissions, similar questions arise. Are we looking at the work of an artist who is content to use his talent ad maiorem dei gloriam? Or is the artist glorying as he exploits and re-forges the iconographic tradition merely as an expression of his own genius?

We can surely assume that Fra Angelico was comfortable in his vocation. What about Donatello? There is no greater sculptor; no one with a more total command of such a range of materials; no one who did more to revolutionise technique. Take his bas-relief, The Feast of Herod. No one has ever captured so much intensity in such a small space. At the other end of the physical scale, there is the Gattamelata, the greatest of all equestrian statues, the subject portrayed in a harmony of strength and serenity: Othello before he met Desdemona. Later, Verrocchio tried to out-do Donatello, and failed. The Gattamelata is Marcus Aurelius. Verrocchio’s Colleoni is Mussolini. Donatello’s best-known work is the bronze statue of David in the Bargello; a religious theme, obviously, but religious conviction? That is much more debatable. For centuries, the received explanation was that the David represented the triumph of civic virtues over Goliath’s barbarism. But ‘virtue’ in any form is not the overriding impression. David is portrayed as a beautiful youth, in as languorous and seductive a pose as is possible when the boy has one foot on Goliath’s severed head. Among Donatello’s contemporaries, it was widely assumed that he was homosexual. This did not appear to upset him, or them. There is no suggestion that he was ever harassed by the authorities. There is no evidence in the David that the allegations were untrue. In this great work, religion seems wholly secularised.

That is not the whole story. In the Opera del Duomo, there is another sculpture. Humbler, in that it is carved in wood, it has never attracted as much attention as the David. But it is an even greater work. In Christian mythology, as opposed to the Bible, it is generally assumed that Mary Magdalene was the woman taken in adultery. Although Christ told her to go and sin no more, she had difficulty in taking His advice. The penitent Magdalene is a frequent theme in religious art and Donatello produced the finest example.

His Magdalene is frail and ravaged beyond her years. Her sins have not been kind to her flesh. She looks as if she does not have long to live. Her arms are stretched in supplication, as if she who once solicited for lust is now soliciting for pity. But that is belied by the beatific expression on her face. It is as if she were ready to embrace death as her final lover – because she is confident in the Redeeming power of the one Man on whom she bestowed a sinless love. It is as if she were echoing the doomed nuns on the Deutschland: ‘Oh, Christ, Christ, come quickly’.

‘I have been a wicked woman’, she seems to be saying, ‘and I have only renounced sin because my body is renouncing me. I am suffering now and suffering deservedly. But none of that matters, because I know that my Redeemer liveth. As I succoured Him in his hour of agony, he will succour me in mine. Beyond sin, there is forgiveness; beyond death, there is joy and eternal love’.

There is no more powerful testament to faith. Every time I see that statue, I echo King Agrippa: ‘Almost thou persuadest me to be a Christian’. The question remains: did Donatello persuade himself?