I SAW the National Theatre's astounding Edward II and the Ashmolean Museum's Bacon/Moore exhibition on the same day.

Towards the end of a visceral production (Joe Hill-Gibbins), Marlowe's humiliated king was thrown to the ground. Roughly shaved and half-naked, he lay face down, deluged with water and soaked to the skin, surrounded by his tormentors; the actor who had played his cast-off lover Gaveston here returned as his sadistic executioner.

John Heffernan's performance had been rheumy-eyed and almost spectral, as if the cloth of gold of his coronation mantle in which he had folded himself and his boyfriend was the only protection between him and the world.

Scribbling notes in 1940 across A sheet of studies for sculpture, Henry Moore observed: "the human body is what we know most about, because its [sic] ourselves & so it moves us most strongly." Hill-Gibbins and Heffernan have taken this to heart, as Marlowe's tragic animal seems to reel "from the boniness of Moore's sculpture . . . to the bonelessness of Bacon's figures" in one evening's pained trajectory.

The critic who wrote that in a review in The Times of a joint Moore/Bacon show at the Marlborough New London Gallery in June 1963 concluded that "these convulsive deformations may still at times seem horrifying." Horrifying, certainly, but is their art tragic?

On a busy Saturday, the Ashmolean was a haven of peace. By rights, this exhibition should have had people queuing out into St Giles for tickets. Fewer than three dozen were in the gallery.

The first encounter from works of both artists selected by Martin Harrison and Richard Calvocoressi is overwhelmingly tense. In the first of several clever coups de théâtre, the in-house designer has Moore's 1956/7 Falling Warrior confront the 1963 Portrait of Henrietta Moraes, one of some ten that Bacon painted from photographs taken by his friend John Deakin.

To say that both are reclining softens the gracelessness of their poses. The juxtaposition of the figures, both of a similar scale, is violent and tense. Both angry figures, trapped and vulnerable, set up a discourse amply explored in this rich and adventurous exhibition. Much like Heffernan in his sodden boxer shorts, these figures are uncomfortable in their own skin.

Maybe it never has been easy to be clad in flesh. The overriding purpose of the incarnation, as George Herbert explores - close to a heresy, but let us not split hairs - in his 1633 poem "The Bag", is that it was not easy for God, either.

He did descend, undressing all the way. . .
And when they ask'd, what he would wear;
He smil'd and said as he did go,
He had new clothes a making here below.

Stripped of all vesture, Moore's ancient soldier and Soho's fallen woman, much like Edward II's crumpled and ruined torso, is only so much awkwardness and real pain. If any of them could speak, the wasted sense of Psalm 6 comes to mind.

The Ashmolean exhibition, which will be seen later in Toronto, however, offers another explanation by way of reference to several drawings by Michelangelo that were presented to the museum in 1846.

The recumbent male figure, the tortured back of a male nude, and, more expressly, the awkwardness of the lumbering figure trying to rise from the ground (all Ashmolean, 1846.55, 1846.42, and 1846.76), offer poses that are almost impossible to sustain. I wanted to leap on to the stage at the Olivier to ask Heffernan how on earth he maintained his broken form, since even Michelangelo's model must have been in great pain.

Both Bacon and Moore took Michelangelo as their master. As if to emphasise this, the Ashmolean includes Rodin's 1863 famed bust The Man with the Broken Nose. The Florentine master looks a real bruiser, and both Bacon and Moore salute him as such.

The sudden comfort of a livid mauve wall and a hint of turquoise on her mattress offer a little repose to Moraes, but her violently contrived (and arresting) posture at once makes clear what true repose means: it is the sleep of the dead or of those about to die.

In the Garden of Gethsemane,the closest disciples of the Christ fall asleep. Vainly they try to shield themselves from the storm that is about to break over them, col-lapsing into the arms of Sleep, the daily reminder of our certain mortality.

Moore's long sequence (1940/1) of the Shelter Sleepers, forms huddled or seated half-dead in the tunnels of the London Underground during the Blitz, owes much to Mantegna and Bellini. They are sleepers who may or may not awake to a new dawn: the victims at Charing Cross and Trafalgar Square in October 1940, and those killed in the direct hits on Bank and Green Park stations (11 January 1941), are hereby commemorated in Moore's sketchbooks.

The central gallery delivers the most powerful coup with nine large canvases by Bacon and six monumental sculptures. The hall is dominated by the great Crucifixion triptych of 1988, in which the 78-year-old Bacon reworked the earlier (and smaller) composition of 1944. Blood is rarely as dense as the background of these scrupulously attended paintings. The forms and the composition would shock any congregation, and for Bacon recall adolescent visits to the abattoir, and later trips to the meat racks of Soho.

The terrifying figures of the avenging fates, the Eumenides, are a far cry from the 1945 maquette for Moore's demure Three Standing Figures, which is now in Battersea Park. Like the three Maries at the foot of the Cross, these draped women look upwards, hopeful and silent, and yet anxious, as if another air raid is about to come.

His King and Queen (1951) derives from Egyptian hieratic figures often as shrine guardians and recall the Moai on Easter Island. Here, in Bacon's last intended painting of Pope Innocent X (1965), the imperious figure sits back at a greater distance, half withdrawn from the regal sculptures placed in front of him. Bacon surprisingly had never seen the original of the Velázquez portrait in Rome, but in this final study he has travelled a long way even from a monochrome photograph.

Neither Moore nor Bacon, who first showed together in a wartime exhibition and then shared a billing at several London exhibitions in 1963, 1965, and the early 1970s, professed a formal faith, but their repeated use of Christian themes goes beyond a common cultural inheritance.

In the last room, opposite a wall of six head studies by both artists, are the three great drawings of the Crucified which Moore produced at the end of his life in 1982. Their overt debt is to Michelangelo, of course; and so the exhibition comes neatly full circle. Moore had long been fascinated by the mystery at Calvary, and in 1954 had apparently planned a sculpture based on a wooden crucifix from Styria, which is now in Ljubljana.

Seeing all three next to each other for the first time, I was left to reflect that the drawings are perhaps among the finest works of Christian art in the last century, a testimony in understatement.

"Francis Bacon, Henry Moore: Flesh and Bone" is at the Ashmolean Museum, Beaumont Street, Oxford, until 19 January. Phone 01865 278002. www.ashmolean.org