“I had rather see a portrait of a dog that I know,” Samuel Johnson once exclaimed, “than all the allegorical paintings they can show me in the world.” The portrait, in his view, was one of the few solid, no-nonsense forms of visual art, a genre with its feet – preferably all four of them – firmly on the ground.

The National Gallery’s entrancing new exhibition, “Renaissance Faces”, furnishes conclusive proof that portraiture always was more complicated than the good doctor liked to think. The show traces the portrait, in post-classical western European art, back to its early fifteenth-century origins in the work of the Flemish master Jan Van Eyck and his followers. It follows the genre’s development and migration, to Burgundian France, to England and – by way of a culmination – to Italy in the sixteenth century. The assembled portraits, of princes and soldiers, popes and bankers, are endlessly fascinating but by no means straightforward. Allegories abound. Skulls lurk, status symbols gleam.

The mood, enhanced by restrained lighting and the sombre battleship grey chosen for the walls of the Sainsbury Wing’s basement galleries, is touchingly intimate. To walk through the seven rooms of the exhibition is to move through a vivid crowd of the dead, to meet the solemn gaze of the men and women of the Renaissance and experience that same, startling illusion of living presence that beguiled them into having their portraits painted in the first place. As the architect, theorist and humanist scholar Leonbattista Alberti remarked, “Painting contains a divine force which not only makes the absent present, as friendship is said to do, but moreover makes the dead seem almost alive.”

The faces revealed, in “Renaissance Faces”, often seem uncannily contemporary. The women of sixteenth-century Venice, immortalised by  Titian and Palma Vecchio, exude filmstar glamour and poise. Intellectual types, such as the humanists Andrea Navagero and Agostino Beazzano, who were painted together in a dark room by Raphael one day in about 1516, stare out at posterity with the same expressions of self-conscious seriousness that modern-day authors assume when photographed for their book-jackets. More down-to-earth Renaissance men and women – exemplified by Giovanni Battista Moroni’s The Tailor, of 1575,or Domenico Ghirlandaio’s wizened old man, embracing his grandson, of nearly a century earlier – have  all the actuality of people encountered on the bus, or in the dentist’s waiting room. Depicted warts and all (and some of their warts are fairly spectacular) they keep their counsel and hold their breath forever.

There is a strong and simple pathos about the exhibition, which stems from the fact that every one of its works is inevitably, among other things, a stark memento mori. These were all people who once moved and breathed, whose feelings were once stirred like those of anyone alive today. But to press up against the glass of history and look into the eyes of its inhabitants is also to be reminded that, in the end, the past really is another country. People who seem at first sight to be just like us, turn out to be nothing of the kind.

The withdrawn, impassive young man depicted in Jan van Eyck’s Portrait of a Man of 1432 – the so-called “Leal Souvenir” – has been posed above a cracked stone ledge inscribed with various phrases including the ancient Greek words “Tum. Theos”, which may be translated as “then God”. He might appear as though gazing into the middle distance but what he is really looking to is the future of his soul. Time will pass, stone will crack, but his piety, to which this small tablet of painted wood bears witness, may gain him eternal life and a place among the blessed. An inscription on another fifteenth-century Flemish portrait plays on the double meaning, to the Renaissance mind, of the word “representation”, signifying both an image and an act of intercession with God. In the case of van Eyck’s picture, image and act are one and the same. The picture is a painted prayer.

Renaissance works of art are rarely innocent of implied meanings and most of the assembled portraits are painted in one kind of code or another. Justus of Ghent portrayed the most successful Italian mercenary soldier of the fifteenth century, Federigo da Montefeltro, in a cramped and low-ceilinged room filled with pregnant detail. The ruler, enlightened despot of Urbino, is accompanied by his son, to whom he reads from a great red leather-embossed book. Federigo wears a gleaming suit of armour beneath his ermine robes. His sword is at his side. The picture is a testament to a self-made Renaissance man’s love of learning and his devotion to splendidly produced, hand-illuminated books (Federigo had a huge library, from which he proudly excluded all printed texts, which he considered vulgar). But it also underlines his ambition to found a dynasty. He is teaching his son, by example, that power without learning, the sword wielded without guile, amount to nothing. Justus painted Federigo from the side, partly to disguise the fact that he only had one eye – the other had been gouged out in a jousting accident – but also to make him look like one of the warlike Roman emperors of the distant past.

When a Renaissance man had himself painted in profile, he might just have been presenting his best aspect to the world, but there was usually more to it than that. It was a way of claiming kinship with the classical past, of proclaiming a determination to emulate the supposed virtue of the ancients. An intense young man painted by Hans Memling in 1475, subject of a small but limpid portrait lent for the occasion by the Koninklijk Museum voor Schone Kunsten in Antwerp, clutches a coin of Nero in his left hand – a visible symbol of the sitter’s humanist interest in antiquity, matched by his keen enquiring gaze. The Italian artist Pisanello actually portrayed his  patrons, the Este dynasty, on coins or medals of cast bronze specifically designed to evoke the Roman coins of antiquity.

In a similar vein, Renaissance sculptors mimicked the chilly realism of Roman portrait busts. The humanist Niccolo Strozzi, carved in marble by Mino da Fiesole in 1475, comes across as a well-fed, jowly, slightly decadent member of the Roman Senate. Francesco Sassetti, consigliere to the Medici, is envisaged by his own portrait sculptor, Antonio Rossellino, as a latterday Brutus. Beady-eyed and circumspect, his shoulders are draped by the suggestion of a toga. Classical reference, here, begins to look like a master-stroke of interpretation. What better model than Brutus for the image of a subtle, self-motivated, reserved, alliance-shifting fifteenth-century Florentine banker?

Subtlety shades into outright secrecy. The humanist author Lodovico Domenichi was pleased that the arcane classical symbols included by the artist responsible for his portrait were difficult to decipher: “I wanted it to be understood by some, and not all”. His attitude was echoed by an adviser to the thuggish Italian mercenary Sigismondo Malatesta, who warned his master against commissioning art that would be too easily understood by “the vulgar mass of people”. That spirit of intellectual elitism was by no means uncommon during the Renaissance. Holbein’s great masterpiece, The Ambassadors, is a veritable crossword-puzzle of a painting, teeming with symbolic allusions to the two French diplomats’ melancholy view of the fractured, post-Reformation world. The unstringed lute spells discord. The skull, symbol of death, rendered in anamorphic perspective on the floor, has itself been painted as a riddle within an enigma – an image which one baffled early director of the National Gallery, Ralph Wornum, believed to be some kind of bony fish.

The ne plus ultra of the coded Renaissance portrait may be the Italian mannerist Giuseppe Arcimboldo’s ingenious depiction of Holy Roman Emperor Rudolf II composed from the fruits of autumn – ruddy apples for cheeks, a pear for nose, berries for eyes and a moustache of corn husks. This blatant bizarrerie was intended to puzzle and amaze, a riddle to test the hapless Habsburg courtier. Only the chosen few were rewarded with the key to its meaning, contained in a poem about Vertumnus, spirit of autumn, by the Roman author Propertius. The fruit-and-veg emperor is a symbol of Rudolf II’s eternally fertile reign, a new Golden Age.

“Renaissance Faces” is itself a cornucopia, charting the ebb and flow of artistic influence across the patchwork of states that constituted Europe during the fifteenth and sixteenth centuries. Northern Renaissance painting is often considered as something of a poor relation to the art of Renaissance Italy, but as this exhibition proves nothing could be further from the truth in the case of portraiture. The most compelling early Renaissance portraits were painted by the first generation of Flemish painters truly to master painting in oils – a medium which, with its unparalleled ability to catch subtleties such as the moisture in the corner of an eye, or the glint of light in human hair, was perfectly adapted to the capturing of a human likeness. Italian painters, from Antonello da Messina in the mid-fifteenth-century onwards, would eventually master oil painting, and with it the art of portraiture; but for quite some time it remained a mode of art associated above all with Flanders.

The quickfire spread of the genre’s popularity is reflected in the geographical span of the National Gallery’s exhibition, which includes outstanding works by the English miniaturist Nicholas Hilliard and the French court painter Francois Clouet, as well as a host of masterpieces by Italian Old Masters ranging from the titans, Raphael and Titian, to less universally feted painters of briliance such as Pontormo and Lotto. Confronted by such an embarrassment of riches, anyone seriously interested in Renaissance art is likely to feel like a child in a sweetshop.