If faith can be said to have fashions, then it has been worn loosely for several seasons. The Christian belief that underlies the great religious paintings of the Renaissance is for many people an alien concept: it can appear, to modern eyes, too structured, too certain, too sentimental. At this time of year in particular, surrounded by painted-by-the-yard Nativities, the faith that brought them into being seems as distant as the age in which they were created.
The German painter Caspar David Friedrich, 1774-1840, is perhaps a type of artist more suited to our times. His Christianity is not insistent but comes wrapped in another - more widely practiced - religion: Nature. He offers the consolations and beauties of both.
Take, for instance, his 'Winter Landscape' (1811), which hangs in the National Gallery. At first sight this bizarre little work is hardly a religious picture at all: a simple depiction of a bare, snow-covered landscape, dotted with rocks and pines, with the incongruous sight of a fantastical church - its spires, which echo the shape of the trees, more Disney than Gothic - just visible through the mist. It is only when you see a pair of crutches lying in the snow and then the figure of a man praying at the foot of a wayside cross, hidden among the trees, that it becomes clear that the painting has a message.
On the face of it the symbolism is clunking: a diagonal line runs across the painting, follow it and the discarded crutches lead to the praying cripple and on to the ghostly church - so adversity through prayer equals salvation, QED.
'Winter Landscape', however, is a more complicated picture than it initially appears: into this small canvas Friedrich has packed in a wide range of concerns and references. Although it works alone the painting is, in fact, one of a pair. The companion picture, which hangs in Schwerin, shows the same cripple in a blasted landscape under a black sky surrounded by felled or dying trees. It is an image of physical and spiritual despair. In the London painting Friedrich reverses every element and gives each a clear symbolic function - the dark sky is now streaked with the light of dawn, the snow is broken by shoots of grass poking through, the dead oaks have become evergreen firs, the desolation has given way to the promise of redemption.
This concern with opposites is at work throughout the picture, in both subject matter and technique: Friedrich contrasts not just the temporal with the spiritual and naturalism with artificiality, but also soft focus against sharp and foreground against background. Most intriguing of all, perhaps, is that the figure in both pictures bears a strong resemblance to Friedrich himself. In 1835 the painter was to suffer a stroke which left him incapacitated until his death five years later; in this painting he foretells his own fate.
Friedrich's faith was a product of his place and time. The son of a candle-maker and soap boiler, he was born in 1774 in Greifswald on the Baltic coast. This part of Germany had fallen to Sweden during the Thirty Years War, which was one reason why its prevailing Christianity was robustly Protestant rather than Catholic. The puritan milieu of his youth helps to explain why the ruined abbeys that feature so often in his work were not for him - as they were for so many of his contemporaries - nostalgic symbols of an earlier, purer faith but of a religion whose moment had passed and which had now fallen into decay.
Just as important as his faith were the intellectual mentors who gave it an aesthetic dimension. The most influential were perhaps Johann Georg Hamann (The Magus of the North) who propounded a view of the world that was based on symbols rather than religious dogmatism, and Immanuel Kant whose Critiques were interpreted by many people as a refutation of the possibility of artistic objectivity.
The effect on Friedrich of this liberating creed of symbolism and subjectivity was to encourage him to set about discovering a new type of Christian art, one freed from the imagery of the Bible and the paraphernalia of traditional religious painting. Not unnaturally for a man of his time he looked for his God in Nature. He was not a fully signed-up pantheist but Wordsworth's lines 'The sounding cataract/Haunted me like a passion: the tall rock, the mountain, and the deep and gloomy wood ...' could have been written by Friedrich to describe any number of his paintings. And while he may not have been a regular churchgoer his faith - and his aim - was specific: 'God is everywhere,' he claimed, 'in the smallest grain of sand. I also wanted to show him in the reeds.'
Friedrich's attitude to landscape painting - traditionally a genre low down on the artistic pecking order - proved controversial. One writer, in a swipe at the type of feeling exemplified by Friedrich's art, bemoaned 'that mysticism, that is now insinuating itself everywhere, and that comes wafting towards us from art and science, from philosophy and religion, like a narcotic vapour'. Others attacked the very notion of presenting allegory through landscape: how could a tree replace the human figure? How could natural forms have meaning without the power of gesture or of symbol?
It is not entirely clear that the painter wouldn't have had some sympathy with his critics since his aim was not so much the legibility of his symbols as to create an atmosphere of devotion, contemplation and religious mystery: he wanted to naturalise the divine. This, however, could not be achieved without effort. 'So as to become what I am,' he wrote, 'I must give myself entirely to my environment. I must melt into the clouds and rocks ... I once spent a whole week in the Uttewalder Grund, among the rocks and pines, and throughout this time I did not encounter a single living soul ... even for me it was a bit much. One ends by falling into melancholy.' Not that Friedrich needed much prompting: he had much to be melancholy about. His gloominess was thought to stem from seeing his brother fall through the ice while skating and drown in front of him (according to some accounts he was trying to save the 13-year-old Caspar David at the time).
It is this accumulation of experience that makes his 'Winter Landscape' so unexpectedly effective. After all, he declared that: 'The artist should not only paint what he sees before him, but also what he sees within him.' This is the real subject of the picture. Perhaps, too, it is not overly fanciful to see the painting as a transformed Nativity scene: the fir-trees taking the place of the stable, the praying cripple standing for the shepherds paying homage, the floating cathedral for the hovering star, the crucifix for the infant Christ - Christmas and Easter in one.
As to whether the modern viewer need see it as anything more than a seasonal landscape with agreeably nebulous religious overtones, Friedrich himself left the question open. He described the wayside cross in another of his paintings as representing 'to those who see it as such, a consolation, to those who do not, simply a cross'.