The Morgan Library & Museum has one of the richest and most extensive collections of illuminated manuscripts in the world. Yet the Morgan rarely flexes its bibliothecal muscle, preferring instead to wow us with a changing sampling of exquisite handmade books that at any given time are among the greatest artworks on view in New York. It is not surprising, then, that when it does choose to focus on a theme and culls together a show drawn entirely from its own holdings — as the Morgan’s curator and department head of Medieval and Renaissance Manuscripts, William Voelkle, has done with “Apocalypse Then: Medieval Illuminations From the Morgan” — that it comes up with an exhibition that is as exhilarating and engaging as it is broad and unique.

The occasion for the must-see show “Apocalypse Then” is not the state of contemporary world affairs but the completion of a facsimile edition of the Morgan’s “Las Huelgas Apocalypse,” whose production required that the Spanish manuscript, to be photographed, be unbound. Right now, still unbound, 50 of the manuscript’s original leaves are installed numerically under glass at the Morgan. The pages, along with an astounding grouping of Spanish, French, Flemish, English, and Russian “Apocalypses” from between the 10th and the 18th centuries, surround one of the new facsimiles, which rests on a table at the center of the gallery and can be leafed through by the public. All of the later manuscripts are a treat, but it is the Medieval Spanish “Apocalypses” that steal the show.

The masterful “Las Huelgas” manuscript, named after the Castilian monastery Las Huelgas de Burgos, where it was discovered in the 19th century, was written by the Asturian monk Beatus of Liébana in 775–76. It is the largest and latest dated (1220) of a 500-year series of Spanish Medieval illuminated commentaries on the Apocalypse, or the Book of Revelation.

The Book of Revelation, the last book of the Christian Bible, was accepted during the Middle Ages as having been written on the Greek island of Patmos by St. John, Christ’s apostle and author of the fourth Gospel. It is an account of the events — including earthquakes, flaming lakes, plagues of human-headed locusts, and falling stars from heaven; as well as the coming of War, Famine, Pestilence, and Death (the four horsemen of the Apocalypse) — that precede Christ’s Second Coming on the Day of Judgment.

In 633, the Book of Revelation was decreed a canonical text in Spain. After Beatus expanded it with a prologue, addendum, and commentaries, it became extremely popular for illustration. This was especially true as the millennium approached, and in the Asturias of Spain, where Christian monks took refuge during the nearly three-century-long Muslim occupation of the Iberian Peninsula (beginning in 711). In their scriptoria, although influenced by the Hiberno-Saxon traditions of early Christian manuscript illuminations, the isolated Christian monks produced some of the strangest and most original, fantastical, and visionary abstract illuminated manuscripts in the Western tradition.

These Spanish “Apocalypses” were extremely important works that introduced popular images, such as Christ in Majesty, the Adoration of the Lamb, and the Madonna of the Apocalypse, and contributed to the widespread use of the Evangelists’ symbols: Matthew as the angel, Mark as the lion, Luke as the ox, and John as the eagle. They also most likely had a strong influence on the flowering of abstract Romanesque art; as well as, in the early 20th century, on Modernist abstraction.

Besides the “Las Huelgas Apocalypse,” the Morgan Library also happens to own the earliest extant version (there are only 27), and the greatest, of these Medieval Spanish “Beatus Commentaries” — the Morgan’s Manuscript 644, popularly known as the “Morgan Beatus,” or the “Beatus of Liébana.”

Meyer Shapiro famously insisted that, if Fernand Léger could only see one artwork in New York that it should be the “Morgan Beatus.” (Right now, I would suggest the same thing.) Illustrated around the middle of the 10th century by a monk named Maius, the revolutionary “Morgan Beatus,” from which two spreads, the “Last Judgment” and “The Vision of the Lamb,” are on view in “Apocalypse Then,” is in many ways a template for the other Spanish “Apocalypses” on view, including the “Las Huelgas.”

The pure, brightly colored illuminations in both the “Las Huelgas” and the “Morgan Beatus” are as straightforward as children’s illustrations or folk art, and they burst forth like mixed bouquets. Purely abstract and as flat and syncopated as Mondrian’s “Broadway Boogie-Woogie,” the geometric illustrations consist of checkerboards, pinwheels, broad bands, targets, figures, beasts, stars, and angels that are all equally frontal, primary, and held competing in the vibrantly colored plane. The paper-doll figures’ faces, bug-eyes, hands, and feet feel tucked in or pasted on; and their drapery and hair are linear, ornamental striations that give dynamic energy and interplay to their bodies but that do not describe or suggest volume. It is as if the energy and prophesies of God — both the threat of damnation and the promise of redemption — are pulsing through the pictures’ veinlike lines and brilliant colors.

Extreme order, clarity, and simplicity of design rule these works. Nothing appears to be in front of anything else, every form vies for immediacy, and each form is clearly delineated and individually present. There are no shadows, and the forms feel more symbolic than descriptive, yet the illustrations honor fully and follow exactly, one-to-one, the words of Revelation. As you move in the manuscripts from form to form — and color to color — every object, every incident, is rendered equal to every other. Every form — be it the Archangel Michael, the Tree of Life, the Seventh Seal, “the sun [which] became black, like a haircloth sack” or “the entire moon [which] became like blood” — is illustrated, following closely the metaphors laid out in Revelation. And, like individual chords, they ultimately add up to, and are subsumed by, the music of the greater whole.

The “Heavenly Jerusalem” in “Las Huelgas,” seemingly without top or bottom, resembles a Disneyland castle and is flattened and splayed out laterally like a child’s fold-up cardboard dollhouse. The damned figures in the “Last Judgment” of the “Morgan Beatus,” suspended in a violet lake, twist and tumble as if in infinite confusion and freefall. The manuscripts, through abstraction, bring both heaven and hell to life. They illustrate a world completely other; one of mystery and of the hereafter — the here-to-come. Fantastical and hallucinatory, they convey a universe in which fairytale and atrocity, the infinitely glorious and indescribably terrible, coexist side by side.

Until June 17 (225 Madison Ave., between 36th and 37th streets, 212-685-0008).