Cranach, Royal Academy, London

I'd guess the best-known image in the Royal Academy's Cranach show is his Bristol portrait of Martin Luther. Painted in 1525, this is so insistently plain – a massing of green and black, with Luther bare-headed in the middle – that it seems impossible it should have any agenda other than literal representation. It is, perhaps, the first great Protestant portrait, balefully rational, unadorned. And yet Cranach's picture is as revolutionary a document as the Ninety-Five Theses Luther had nailed to the door of Wittenberg's Schlosskirche eight years before.

To see why, look at the pictures Cranach was painting a decade earlier – his Mystic Marriage of Saint Catherine, say. The difference between the two works marks the shift from Catholic mysticism to a Protestant emphasis on Man. In the bleak theology of Martin Luther, there was no buying your way into heaven via the intercession of the saints. Faith alone justified salvation, which put the finger on the individual. And Cranach's Luther is correspondingly individualistic, certainly compared with his clothes-before-character portraits of the Saxon Elector, John the Steadfast, and his son.

His plain-spoken portrait of Luther differs from these, and from the Mystic Marriage, in another way as well. With their Gothic drapery and Eyckian rendering of cloth-of-gold, Cranach's earlier pictures look back to the certainties of court patronage and court painting. By contrast, the Luther portrait is naturalistic and democratic, and with good cause. Among its austerities, Protestantism shunned graven images as aids to worship, and this spilled over into an iconoclastic mistrust of pictures in general. A portrait of Martin Luther was, by its subject's own lights, a potentially wicked thing.

Nearly three centuries later, Jean-Jacques Rousseau would face French painters with this same quandary. Painting was illusionistic and illusions were lies: so how could one paint morally? Artists such as David arrived at the same answer as Cranach. Although painting was wicked, engraving was not. It was democratic, because the images it produced were cheap and could be handed around. Engravings educated the eye rather than fooling it. Rousseau had trained as an engraver; David began making canvases whose massing and strong silhouettes echoed those of engraving. Luther's vernacular Bible of 1522 – a cornerstone of the Protestant Reformation – had been illustrated with woodblocks; Cranach was a gifted printer, The Penitent Magdalen in Ecstasy and its kind being among the most powerful in this show. And so, like David, he echoed his master by painting like an engraver.

You see this in his Luther portrait, effectively composed of three strongly delineated parts: cloth, background, face. The picture's size, too, makes it print-like and simple. Luther stands bare-headed before Cranach as Protestant Man stood before his God, personal and unmediated. There is no deconstructing to be done, no working out of class or rank, no mystery. Hier stehe ich, says Luther's portrait, und kann nicht anders: Here I stand, and can do no other.

And yet, of course, this lack of illusionism is itself illusory, simplicity in paint being just as phoney a thing as complexity. In many ways, Cranach's genius lay in his capacity to square just such circles as these. When his patrons demanded Protestant austerity, he reached for strong forms and a two-part palette; when they wanted Roman luxury – and, for all his Protestantism, Cranach never turned away a Catholic patron – he went for cloth-of-gold and non-rational light. He could, by turns, paint copybook images (such as The Golden Age) that looked back to the Middle Ages, and portraits that looked forward to the Enlightenment.

Towards the end of his long life – he died in 1553 at the great age of 81 – Cranach's interest lay in making erotic images whose voluptuousness was a broadside at the duplicity of women. Yet their purpose was to titillate. That paradox still gives subjects such as his Venus a vaguely unpleasant power, certainly enough to have her banned, initially, from London Underground as a publicity poster for the RA's show. Venus evokes the very sin she allegedly warns against, just as Luther embodies Lutheranism. Seeing the two works together makes you feel that Cranach was probably not a very nice man, although niceness has never been a prerequisite of greatness.

Royal Academy, London W1 (020-7300 8000) to 8 June