In 1967, the poetry magazine Agenda published a special David Jones issue, including a number of unpublished fragments of his work. The first of these was the brief piece entitled “A, a, a DOMINE DEUS”, often reprinted as Jones’s most poignant statement of his sense that the world of technology was making the writing of poetry – and indeed the other arts – impossible: “I have watched the wheels go round in case I/. . . might see the Living God projected/from the Machine . . ./my hands found the glazed work unrefined and the terrible/crystal a stage-paste”.

He had elaborated on this two decades earlier in a note addressed to the doctor who was treating his paralysing depression and anxiety. We are living, he wrote, in a culture where objects are thought of in terms of their usefulness. An electric light bulb is designed to illuminate human dwellings or workplaces; if an artist wants to evoke something about light more generally, the light bulb is not a good metaphor, because it is merely a functional object. It is what it is because of the job it has to do. But we need images that are allowed to resonate more freely because they are not determined in this way – fires, stars, the sun. How then does the artist avoid “a kind of invalidity”, a corrupting distance from the actual world of his or her experience?

Jones often wrote about “the Break”, the cultural moment somewhere around the beginning of modernity when the European world-view shifted decisively. Instead of a world where things were unique but linked by an unimaginable density of connection and cross-reference, we had created one in which things were unconnected but endlessly repeatable and where everything could be exchanged in the market for an agreed equivalent: above all, for money. Jones saw his work – both as a visual artist and as a poet – as a sustained protest against the Break and an effort to show that the older picture could, after all, be brought to life.

Born in 1895, he had family roots that helped to shape his interests from the beginning. His mother’s father had been a London shipwright and his father’s origins were in North Wales. Both Wales and London kept a central place in his imagination throughout his life. It was not surprising that when the First World War broke out, he enlisted in the 1st London Welsh Battalion of the Royal Welch Fusiliers. His 1937 masterpiece, the astonishing book-length poem In Parenthesis, describes the experience of foot soldiers in the First World War, revisiting his own experiences up to and including the disastrous engagement at Mametz Wood in July 1916. Jones was wounded in the leg during the battle (a wound described by the medical orderly as “a beautiful blighty” – serious enough to get him off the front line, yet not life-threatening). But he was back in the trenches in a matter of months.

The traumas of war stayed with him to the end. In Parenthesis, which he struggled with painfully over many years, is one of the most unsparing accounts of the life of infantry soldiers in the trenches and of the horrors of the Somme; but at the same time it meditates on any number of connections – echoes of conflict, from Troy to the struggles of the British against the Saxons in the 6th century to Malory’s Arthurian narratives, and, woven through it all, the founding act of bloodshed that is the death of Christ. Jones was raised an Anglican, but by the time he wrote In Parenthesis he was a Catholic, and believed passionately that the Church’s sacramental theology was what made sense of a world of symbolic connection, where nothing existed as an atom but where everything enriched the perception of everything else. For him, all art rested on the conviction that God had made a world of endless cross-reference, and that humanity was most fully human when it acknowledged this. Art was humanity doing what only humanity could do.

Thomas Dilworth’s welcome (and superbly produced) biography will clearly be the point of reference for Jones’s life for a long time to come. Dilworth has already written extensively about Jones, most recently a full and valuable account of the wartime years, and his research is exhaustive. He quietly corrects a number of errors in earlier biographical sketches and provides a wealth of detail at every stage – and he tells us that this substantial book is only part of a longer document that he intends to publish online. In all the detail, it is hard to pick out a single thesis; but in so far as there is one, it is that Jones is “the foremost native British modernist”, as Dilworth claims in his concluding paragraph.

This may sound strange, given what we know about “the Break”. But in fact, Jones himself believed that the modernist, post-impressionist aesthetic was a decisive break of its own kind – a break with representation as a sort of substitution, a recognition that a work of art is a thing in which something else is allowed to come to life, in a new medium: a picture is the scene or the human figure existing in the form of paint, as the Mass is the flesh of Jesus existing as bread. He insisted that his Catholic conversion began with his artistic conversion, and tried persistently, in his superb essays as well as his artistic output, to show what this meant.

The artistic conversion was dramatic enough. Dilworth reproduces some of the technically skilful and aesthetically awful work of Jones’s early art-school days, as well as some startling propaganda pictures from the war years: languishing virgins being threatened by hairy medieval Teutons, and so on. Jones needed to rediscover the extraordinary talent of his early childhood, when he produced sketches of a delicacy and vigour that foreshadow the very best of his mature work. Immediately after the war, back at the art school in Camberwell, he let his imagination be opened up by a variety of new impulses, ranging from El Greco to Samuel Palmer and Pierre Bonnard.

But Jones’s distinctive touch as an artist came to life when he threw in his lot with his fellow Catholic convert Eric Gill. He shared the life of the Gill family frequently for nearly a decade, in both Sussex and the Welsh borders, imbibing Gill’s distinctive artistic philosophy and gently but steadily distancing himself from it, and was for a while engaged to Gill’s second daughter, Petra. Gill mocked Jones for continuing to paint watercolours, insisting that carving and engraving were intrinsically more serious matters because of the manual work involved: watercolours were just decorative, the worst possible thing for a work of art to be, in his book. The Gill circle was a crucial stimulus for Jones, but ultimately one that allowed him to sharpen up his own understanding rather than adopt an orthodoxy. The watercolours, gouaches and engravings of the 1920s show a striking confidence. In 1928 he was nominated by Ben Nicholson for membership of the “7 & 5 Society”, probably the leading group of artistic innovators in 1920s Britain.

Jones’s acute and recurrent depression and worsening anxiety held back his output in the 1930s, though he struggled through to the completion of In Parenthesis. The later visual works – drawings, paintings, inscriptions – display an exceptional range of idioms and are increasingly characterised by abundant detail that is of filigree precision as well as unusual fluidity. There are religiously themed pictures: Vexilla Regis (1948), the great symbolic tree in the forests of post-Roman Britain standing for the cross as a sort of world-tree; the Welsh hill landscape framing the Annunciation in Y Cyfarchiad i Fair (1963), with its abundance of exquisitely observed small native birds. There are the “calix” paintings of glass vessels holding flowers, which deliver an effect of profound translucency. There are the inscriptions of Latin, Welsh and English texts, a unique corpus of work in which he defined a new approach to “monumental” lettering as an art form. These are perhaps the lasting legacy of his apprenticeship to Gill, yet they are anything but derivative.

In the middle of all this, in the postwar period, he continued to write, producing another unclassifiable poetic masterpiece, The Anathemata (1952), an exploration of both personal and cultural history, with the events of Maundy Thursday and Good Friday at the centre of everything. Other “fragments”, many of them very long, were worked on over years but never found their connecting thread; most of these were not published until after his death.

Dilworth provides a comprehensive account of Jones’s struggles with mental health. He was fortunate enough to find a sympathetic therapist who strongly encouraged him to keep working; but later on, a formidable regime of antidepressant and other drugs left him less able to focus – “groggy and slow”, as he said – and his productivity declined sharply. A temperamental indifference to social encounters combined with tormenting agoraphobia to make him ever more of a recluse in a succession of north London boarding houses and nursing homes until his death in 1974.

Yet his friendships were immensely important to him – friendships with members of the lively and critical world of Catholic artists in the 1920s, with younger artists and writers, to whom he was unfailingly generous, and with the two young women, Prudence Pelham and Valerie Wynne-Williams, who were the recipients of his strongest (but unconsummated) attachments. The breaking of his engagement to Petra Gill had been a great trauma, and his lifelong celibacy seems to have been the result both of this shock and of a deep-seated conviction that his artistic vocation could not accommodate ordinary family life.

He was a wonderful letter-writer; anyone wanting to get to know Jones should start with Dai Greatcoat, the selection from his letters published in 1980 by his friend René Hague (Gill’s son-in-law). Funny, ­affectionate, eccentrically learned, curious, irreverent and sad, they give a good sense of why Jones was so deeply loved by those who knew him. He viewed the world – and his own work and calling – with a gentle and vulnerable bafflement, but also with patience and humility. He seems to have had no malice in his make-up.

Dilworth does not, however, shirk the embarrassing fact that Jones expressed a measure of sympathy for Hitler in the 1930s. This should not be misunderstood. What Jones says is that, having read Mein Kampf, he feels it is almost right, but ruined by hatred and racial triumphalism. Hitler appears to him more appealing than most of his opponents, who represent international finance and impersonal bureaucracy, or Marxist collectivism. He later admits that he was simply wrong. But it is a revealing wrongness: he accepts at face value a rhetoric that opposes the market, and he seems to see Hitler’s passion and violence as at least a more honest response to national or global crisis than the “business as usual” of mainstream politicians. And how far are Hitler’s “opponents” being tacitly understood as the cosmopolitan financiers of anti-Semitic myth? Dilworth does not absolve Jones for dipping his toe into this swamp; but he does note that Jones was – more than many of his Catholic colleagues – intolerant of the anti-Semitism of much traditional Catholic thought and shocked by the persecution of the Jews in Germany. It is another sidelight on his fundamental artistic problem: a disgust with managerial, commodified mod­ernity that, in his case as in some others, can make a quite different anti-modernity, the fascist refusal of public reasoning and political pluralism, fleetingly attractive.

The other delicate issue that Dilworth handles carefully and candidly is whether Jones was aware that Eric Gill had sexually abused two of his daughters (including Petra). His conclusion is that it is very unlikely, and this is almost certainly right. And yet, looking at Jones’s haunting painting of 1924 The Garden Enclosed, with its depiction of himself and Petra embracing awkwardly, Petra apparently pushing him away, with a broken doll lying on the path behind her, it is hard not to believe that he intuited something deeply awry somewhere. The background presence of Gill’s omnivorous sexual appetite can hardly not have been a further complication in an already complicated relationship.

Jones’s reputation has probably never been higher. There have been several important exhibitions in recent years and Dilworth’s assessment of his standing among British modernists is increasingly shared. His thoughts as an essayist on theology as well as aesthetics have been increasingly influential. This biography is a landmark. It would be good if it stirred an interest not only in Jones as an artist and poet, but in the questions he faced about modernity: what happens to art in a culture where each thing is no more than itself, or its market price?

"David Jones: Engraver, Soldier, Painter, Poet" by Thomas Dilworth is published by Jonathan Cape (432pp, £25)