AT THE heart of the recent British Museum exhibition of its Spanish drawings, intelligently staged and brilliantly catalogued by Mark P. McDonald (From Renaissance to Goya, 2012), were seven by the Andalusian artist Bartolomé Esteban Murillo (1617-82).

Early in 1660, Murillo and Francisco Herrera the Younger jointly established a drawing academy in his native Seville, although Murillo later left (possibly prompted by his wife's death in 1663) to teach on his own. Among those who came to study in the Royal Academy of Fine Arts of St Elizabeth of Hungary were the sculptors Bernardo Simón de Pineda and Pedro Roldán, who often worked together on constructing the densely decorated altarpieces and retablos that so characterise churches in the Spanish Baroque era.

Murillo is best-known for his religious paintings and his genre scenes of ragamuffin boys and scenes of childhood, which were last showcased in an exhibition at Dulwich (Arts, 4 May 2001). But it is in his drawings that we sense the delicate movement and subtle composition that make his work so immediately appealing. Unlike the paintings, which to some can appear superficially impossibly sweet and cloying, the drawings show his capacity as a draughtsman and his skill as a composer.

When Alleyne Fitzherbert, formerly Lord Lieutenant of Ireland, was sent as British plenipotentiary to Spain in May 1790, he collected many works of art and drawings, of which 60 sheets by Murillo later came to the British Museum. There might have been many more but for a disastrous fire at his London house in July 1797. When his diplomatic ploy proved successful, Spain renounced claims to the north-west coast of America, and his friend, Commander George Vancouver RN, who first sighted a volcano from the Pacific shore, named it Mount St Helens for his newly ennobled friend.

Among the drawings shown at the BM was a sketch for the great altarpiece of one of the patron saints of Seville, St Isidore. The painting was donated to the cathedral, with a pendant picture of his brother, St Leander, in 1655. It immediately found favour with the cathedral chapter, and hangs to this day in the main vestry.

Seville had been one of the largest ports in Europe, deriving its prestigious wealth from the trade with the New World and its safety from invasion as an inland port. All goods imported from the Indies had to pass through the Casa de la Contratación, in the Alcázar. Even Miguel de Cervantes, who had earlier worked as a purveyor for the Spanish fleet, found work there as a tax collector.

Even though the trade monopoly with the New World was broken, and Cádiz was granted equal access to the market, Seville uniquely possessed ecclesiastical jurisdiction over the Spanish Americas. It had long been an archbishopric (from the fourth century), but the income to the diocese increasingly came from the new territories overseas. To be "as rich as Potosí" was to enjoy the fabled wealth of the Bolivian silver capital; and Lima was known as "New Seville".

As this suggests, ideas and art travelled in both directions. In the mid-17th-century Church of San Agustin in Marchena, an important former Roman colony in the province of Seville, Bartolomé Zumbigo intruded elements of Iberian-American Baroque with decorative plasterwork friezes with monkeys and Aztec angels in a veritable forest of dense South American foliage. This is a far cry from the restrained classicism of his Capuchin church in Toledo, and still comes as a surprise today.

Architects and painters from Spain were dispatched to the new colonies to decorate convents, churches, and palaces. The Andalusian design of churches of mudéjar origin, with a single elongated nave shaped into a cross with a transept, became widespread in Guatemala, Peru, and Mexico.

The common cause of Christianity linked the two worlds. When the first American saint was canonised (St Rosa of Lima, by Clement X in 1671), the Dominican convents of Seville, of the Madre de Dios, San Pablo, and Regina Angelorum, all held festivities in her honour.

But trade and shipping also brought their own problems. It is likely that the Great Plague of 1649 came with ships from North Africa. Despite localised quarantine, more than one quarter, and possibly half, of the population of Seville had died in three years. Estimates suggest that more than 60,000 civilians died.

In her 2007 doctoral thesis, Lisa Duffy-Zeballos has convincingly suggested that the lasting effect of this plague was a desire by the sevillanos to ensure that, in the face of death, their piety became more public and their cults more distinctive. What Dr Duffy calls the "Baroque culture of prayer" finds its fullest expression in Murillo's paintings of saints which were no longer the idealised figures of Counter-Reformation propaganda, nor the realistic barbarous images associated with later Caravaggism. The point is equally well made by Sergio Ramírez González in his 2011 book on the iconography of religious orders in Baroque Andalusia.

Murillo was born and baptised in Seville, the youngest of 14 children. When his parents died, he was raised by an older sister, and at the age of 15 he gained permission to travel to the New World, where he had close family. Whether he went is not known; for he was soon apprenticed as a painter in Seville.

He qualified in 1639, and married a silversmith's daughter in 1645; but, for the last 20 years of his life, he was a lonely widower. At the time of his sudden death (as a result of a fall, paintbrushes in hand, from a scaffold above the high altar in Cádiz), only the youngest of his surviving four children was with him.

Both the artist and his wife had survived the plague, possibly because the area in which they lived was inland from the river in the former Jewish quarter; the three churches with which he had most to do, Santa Cruz, Santa María la Blanca, and his namesake Church of San Bartolomé, were all former synagogues that had been first adapted to become mosques and churches.

Murillo seems not to have travelled widely; in 1651, he can be found in Marchena, where a rarely seen painting of the annunciation in the enclosed Convent of the Discalced Mercedarians of San Andrés has been attributed to him. If it is by his hand, it would demonstrate his early knowledge of the Dutch printmaker Hendrik Goltzius and the paintings of Agostino Carracci, and his willingness to work for a newly founded community, as the Mercedarians came to Marchena only in 1637.

Other than that, his only securely documented trip outside of Seville occurred nine years after the plague, in 1658, when he was at the court in Madrid. There he could have become familiar with the rather different taste that evolved from artists who knew the work of Titian and the Venetians so much admired of Charles V and Philip II, and that of Rubens. Certainly, his own style altered.

By concentrating on Canon Justino de Neve y Chaves (1625-1685), the Dulwich exhibition undertakes to do two things. It shows the benefit of ecclesiastical patronage, and explores the gift of friendship, which brought them together.

Neve came from a family that had made its money in the Spanish Netherlands. In 1646, six months before he became coadjutor to one of the canons of Seville Cathedral, he was ordained as a subdeacon. The tablet in the nave of the cathedral records that he served for 40 years as a canon there. He and Murillo became close friends, and Neve was one of the artist's executors.

As Visitador de Capillas from 1657, Neve was responsible for inspecting the city churches and chapels that were dependent on the cathedral. These included the renovated Church of Santa María la Blanca, designed by Pedro Sánchez Falconete in 1657, for which Murillo later provided four lunettes, at the east end of both side aisles and over the transept crossing.

The church is currently closed for restoration, but, amid the shambles of the work of cleaning, it is possible to see where the four canvases (1664-65) once hung before they were expropriated by the French and taken to Paris.

Befitting a church dedicated in honour of Our Lady of the Snows (feast day: 5 August), the two larger pictures depict the vision, during the papacy of Liberius (352-366), that led to the building of the church of Santa Maria Maggiore on the Esquiline hill in Rome, where the miraculous snowfall had marked out the footprint. Neve arranged that the refurbished church be opened amid great festivities on 5 August 1665. Murillo loaned paintings that decorated a temporary altar set up outside the church for the occasion.

When these lunettes were last seen in London, at the Royal Academy in 1983, the gaudy neoclassical spandrels made for them by Percier while they were in France had been kept in place. For the exhibition, that surrounding The Dream of the Patrician and his Wife has been covered over, so that the effect in the chapel-like interior that has become the Dulwich Picture Gallery is much more harmonious.

The two former side lunettes are The Triumph of Faith (Buscot Park) and The Immaculate Conception (The Louvre). In both, worshippers are depicted at either side of the central image, silent encouragers to our faith. Although the doctrine of the immaculate conception first developed in the Latin West in Anglo-Saxon England, it gained currency in Spain and Italy only in the 17th century.

In 1671, Pope Alexander VII's Apostolic Constitution Sollicitudo Omnium Ecclesiarum officially allowed the doctrine. Murillo returned to paint the scene several times after painting the lunette. To the right of the Virgin is a small group of worshippers, prominent among whom is Canon Justino.

We know what he looked like, however, because of the portrait that Murillo undertook at the same time, at his friend's request. It has been in the National Gallery in London since 1979, and is, arguably, the finest picture on show. He can also be spotted in a contemporary print, shown at the BM, that commemorates the cathedral festivities for the canonisation of King Ferdinand III in 1671, when a temporary structure with a processional figure by Roldán (still in the cathedral vestry) were built in the royal chapel.

In the portrait, Neve appears as both patron and priest. This is particularly fitting, not least as Neve, who was an assiduous collector of paintings (he had some 170 at the time of his death), commissioned important works from Murillo for himself and for the institutions in Seville with which he was linked, most importantly his charitable hospital, and the cathedral, where he commissioned Murillo to work in the baptistery chapel and the chapter house.

The cathedral has exceptionally loaned The Lord's Baptism from the baptistery, but (for all too understandable reasons) not its Immaculate Conception from the elliptical roof of the chapter house.

The Hospital for Retired and Pilgrim Clergy, the Hospital de los Venerables Sacerdotes, which Neve founded in 1673 to care for indigent clergy, recently hosted this exhibition in one of the lower-floor infirmaries.

The large altarpiece of the Immaculate Conception of Los Venerables, which had been looted for Napoleon by Marshal Nicholas Soult c.1810, had been commissioned by Neve, some time before 1665, presumably soon after the pope allowed the Virgin to be accorded the title, banned by the Inquisition as recently as 1644.

It was, therefore, one of the first images for the newly sanctioned cult. Neve was proud enough of it to display it at the reopening of Santa Maria La Blanca. It was bought from his estate in 1686, and set over the south altar of his hospital church alongside the Penitent St Peter.

The Immacolata dominated the Piccadilly show, where it appeared with Murillo's much earlier versions of the same subject, from the Church of San Francisco in Seville, and from El Escorial. Although it is a pity not to be able to see it next to that from the cathedral, we can appreciate how Murillo makes her figure stand triumphant in simple radiant glory. There are no traditional Marian symbols of the kind that often clutter such altarpieces, as these are on the surrounding frame.

The altarpiece was finally returned to Spain in December 1940 (in an unlikely art exchange between Petain and Franco), when it became at once a symbol of a new emerging Spain. Visitors can now see it in its original frame, carved with its Marian symbols, which has remained in the hospital chapel since the French cut out the canvas. One prays that the Prado will now send it back to the Venerables.

The most extraordinary and the most fragile work in the show is a small nativity (38.3 × 34.2cm), painted on brittle obsidian (Houston, TX). It is one of three devotional images that Neve owned in which the milky veining on the vitreous stone becomes the background. (Sadly, the Louvre has not sent the other two, Christ at the Column and The Agony in the Garden.)

Neve, who at 40 appears to be a man of the world in his portrait, presumably used them in his daily prayer, as canons of the cathedral were bound to meditate seven times a day on the Passion. Small as it is, it repays close observation, and allows us to give thanks for the friendship that produced it, and which has afforded such a spellbinding show.

The first-rate catalogue has already served the exhibition in Madrid and Seville, and here is accompanied by an extensive essay on the Murillos in the Dulwich collection, written enthusiastically by Dr Xavier Bray, the curator.

"Murillo and Justino de Neve: The Art of Friendship" is at the Dulwich Picture Gallery, Gallery Road, London SE21, until 12 May. Phone 020 8693 5254.

www.dulwichpicturegallery.org.uk