Something strange happened in the Renaissance. The deities of Greece and Rome had been proper gods with temples, rituals, priests and worshippers to go with them. They had disappeared only when Christianity took their place. Yet in the Renaissance, when the gods and goddesses of the ancient world re-emerged, they were no longer part of a religion, they were something else. What exactly?

It is tempting to suppose that the revival of classical mythology was the result of humanist scholarship and the rediscovery of ancient texts. Yet this is not really true, because the most important literary sources for artists were not classical works but vernacular romances and reprints of medieval paraphrases of Ovid that often have little relation to the original text. Even in the mid-16th century when mythographical handbooks of various kinds start to appear, they are often written for, and sometimes by, artists themselves.

Although the revival of interest in antiquity took many forms, the reanimation of pagan mythology is something that took place within the sphere of the arts. And there mythologies emerge first in minor decorative forms - wedding chests and jewellery boxes, the painted plates used in country villas - in the statues for fountains and gardens, and in the decoration of bathrooms and bedrooms. In literature, mythological themes are more common in occasional verse than longer poems; in the theatre, they initially appear as intermezzi, musical diversions staged between the acts of the play, rather than in the drama itself. Mythology did not demand sustained attention and it was not considered suitable for serious public contexts; if it served as more than a filler, it belonged in the private, often female sphere, where people sought undemanding relaxation.

For the contemporary equivalents, you would probably need to think in terms of the style section of the newspapers. But if classical mythology was packaged like the glossies, its content was strictly tabloid - incredible tales and the sexual indiscretions of celebrities: how Jupiter raped me disguised as a woman; pregnant woman stripped at swimming pool; rapist's wife beats up victim; son nearly kills mother in the form of a bear; how I went into outer space. And that's just the story of Callisto - the pregnant nymph whose discovery among the chaste followers of the virgin goddess Diana was the subject of paintings by Titian and others.

But, rather as Jerry Springer transfers easily from late-night TV to the National Theatre, so pagan subjects gradually migrated to more prestigious contexts. By the 17th century, mythology had moved from the intermezzo to opera, the epigram to epic, the frieze to ceiling, and from the gardens of country villas to the state apartments of princes.

Throughout this process, mythological imagery never challenged Christianity directly. No one took down altarpieces or statues of the saints and replaced them with pagan idols. The ancient gods crept back into western culture as fashion accessories and garden ornaments, without anyone taking them seriously. In the end, the result was almost as destructive. There had not been much place for falsehood in the Christian world - cultural products were all meant to contain some truth (even if it needed an allegory to extract it) and those that did not were banned. But the spread of mythological decoration created a tolerance for fanciful imagery.

The effect was corrosive. It made it more difficult for people to distinguish between truths that they were meant to believe, and fictions that they were meant to dismiss as a joke. For the sceptics of the Enlightenment, the ubiquity of the pagan gods was proof that widely disseminated traditions could simply be false, and it was not long before the same reasoning was applied to Christianity and the ancien regime as well. It was, in effect, secularisation through decoration.

But why did the rebirth of the ancient gods transform the culture in a way that, say, 18th-century Chinoiserie, or even the 19th-century Gothic revival, did not? Was it because the superior technical accomplishments of Renaissance artists constituted such a persuasive argument for classical themes? Hardly. Mythological subjects were rarely the inspiration for artistic innovation. If anything, Renaissance naturalism was driven by religious imperatives - the desire to experience the living presence of Christ, Mary and the saints in and through their images - while mythological themes were routinely treated in a way that was flatter and less realistic.

The success of classical mythology in early modern Europe had as much to do with politics as art. Mythological imagery spread after the artistic and intellectual renaissance was well established: from the second quarter of the 16th century in Italy, and later still in northern Europe. The political context in which it developed was not republican Florence, but the imperial territories secured for the Holy Roman Empire by the Habsburg victories in Italy in the late 1520s. The Holy Roman Emperor Charles V was identified with Jupiter, the king of the gods, and flattered by the rulers of north Italian states with depictions of Jupiter defeating his enemies and making love to mortal women.

Republics continued to prefer allegories but, over the next century, princely states all over Europe developed iconographies in which the ruler was identified with one or more of the gods, culminating in the almost total identification of Louis XIV and the sun-god Apollo. Classical mythology was not just the wallpaper of the leisure classes; it was simultaneously the propaganda of absolutism. This does not mean that every little trinket and fountain carried an ideological message, but, taken collectively, mythological subjects formed a continuous web of fantasies at whose centre was the imagery of royal power.

For example, when Michelangelo presented to his young friend Tommaso de' Cavalieri a drawing of Ganymede carried off to Olympus by Jupiter's eagle, it was a private image inspired by his love for the young man and the lofty ideals he hoped they would share. He certainly was not thinking about contemporary Italian politics. Yet a few years later, Michelangelo's image of Ganymede was used by a later artist as an allegory of the Battle of Montemurlo, where Charles V (represented by the eagle) helped Duke Cosimo de' Medici (represented by Ganymede) achieve victory against his enemies.