As the four horsemen of the Apocalypse galloped across fourteenth-century Europe, never had the world seemed more worth leaving or God more worth having. Not only did this era produce mystics of the stature of Meister Eckhart and his disciples, Henry Suso and John Tauler. More remarkably, ordinary men and women thirsted for God and mystical doctrine with a fervour not seen before or since. Did the call to forsake all creatures – to sink into the ‘fathomless abyss and silent desert of the Godhead’ – gain urgency from the unending wars, famines, pestilence and death that rained down on a world already drenched in religion? Merely asking the question risks reductionism. Yet this unprecedented lay interest in an austere mystical piety, centred on detachment from self and creatures (Abgeschiedenheit) and total abandonment to the will of God (Gelassenheit), bespeaks a Church profoundly and doubtless rightly disillusioned with human effort.

The social context of fourteenth-century mysticism is assumed rather than analysed in the latest volume of Bernard McGinn's ever-expanding history. Volume iv, which was originally meant to cover all major mystical texts of the fourteenth and fifteenth centuries, now devotes 700 pages to Germany alone, where Eckhart's influence, like that of Francis a century earlier, launched a spiritual revolution. After a preliminary section on Albert and Thomas and the recovery of Neoplatonic texts, McGinn devotes four hefty chapters to the great Dominican and his disciples. A noted Eckhart specialist, he characterises the master's most distinctive and influential teaching as ‘the mysticism of the ground’. This ‘explosive metaphor’ provided a way to elaborate the ‘union of indistinction’ between God and the soul – not a union of two loving wills, as taught by twelfth-century Cistercians and Victorines, but an ontological oneness in which no distinction between Creator and creature remains. God's infinite, unknowable ground (Grunt) or abyss (Abgrunt) is ultimately one with the soul's ground, that existential core beyond thought, feeling and will whither the mystic strives to return – or rather, ceases striving, to become perfectly free, empty and passive while God alone acts in him. Yet this inner emptiness does not lead to quietism or even the traditional privileging of contemplation over action. Rather, the soul that has truly ‘broken through’ selfhood to attain the ground can remain active as God's instrument in any mode of life. In this respect, clergy have no advantage over laity.

McGinn offers a detailed, technical account of Eckhart's thought, not neglecting his Latin works but paying special attention to his evocative German vocabulary, which would remain ubiquitous for centuries. This language is repeatedly described as ‘daring’, in part because of its propensity to shock non-mystical believers (Karl Barth's God, for example, is indeed ‘wholly other’ than Eckhart's). But the daring also arises from the mystic's proximity to the ‘autotheism’ of Marguerite Porete, burned as a heretic in 1310, and the movement known as the Free Spirit heresy. Only a razor's breadth separates the two. Eckhart, who died under investigation for heresy in 1328, escaped posthumous condemnation (though many of his teachings were censured) only because of his declared willingness to submit to the Church. Beneath the main current of McGinn's study, which flows from Eckhart through Suso and Tauler to the popular diffusion of their message, suspicion of heresy runs like a dangerous undertow. It even became de rigueur for writers who taught indistinct union to protect themselves by inserting an obligatory polemic against Free Spirits – denounced as antinomians who used their liberty to excuse lives of ease and comfort. Surprisingly, McGinn argues that the notorious dialogue ‘Sister Katherine’, in which a beguine exults that she has become God and teaches her confessor to imitate her, was not a Free Spirit tract after all. For modern scholars, as for medieval inquisitors, such discernments are delicate.

Suso and Tauler, while sharing Eckhart's mysticism of the ground, carefully distanced themselves from his more problematic formulations and added emphases of their own. A brilliant stylist, Suso created an idealised self-portrait in his figure of the Servant of Eternal Wisdom, whom he presented as an exemplar of devotion to the passion and the holy name. In dialogue with his disciple, Elsbeth Stagel, Suso represents spiritual maturation as a turn from savage self-torture to voluntary acceptance of persecution. He was by far the most widely read of these mystics, but it was Tauler who, thanks to Luther's admiration, retained his popularity even among Protestants. In that gifted preacher, who unlike Eckhart and Suso never pursued an academic career, we glimpse a growing split between scholastic theology and mysticism. Throughout the period many now-forgotten writers tried to heal the split by systematising mysticism itself, writing treatises on the four forms of preparatory grace or the thirty-six modalities of divine love.

One of the most fascinating, little-known parts of McGinn's narrative recounts the diffusion of mystical theology through alternative media – poems, letters, saints' Lives, visions, vernacular sermons and visual art. An outstanding witness in the last category is the Rothschild canticles, a miniature prayer book owned by a fourteenth-century nun, which conveys the paradoxes of apophatic Trinitarian mysticism through paintings of stunning originality and beauty. From this priceless volume to inexpensive broadsheets and homemade devotional drawings, a plethora of images aimed to mediate union with God. (Music is not included, but could motets on the Song of Songs be considered just as mystical a genre?) The section culminates with Rulman Merswin, a converted banker who used his fortune to found a kind of mystical commune, then promoted its agenda by ghost-writing tracts in the name of a fictional guru, ‘the Friend of God from the Land Above’, who supposedly initiated him into God's mysteries. Although McGinn ends with another brilliant theologian, Nicholas of Cusa, somehow it is this fourteenth-century Carlos Castañeda who most memorably typifies the stirrings of the age.