She travelled regularly within Europe, primarily to Switzerland, France and Italy, where she pursued her interests in art and Catholicism, visiting numerous churches and monasteries.

Initially an agnostic, she gradually began to acquire an interest in Neoplatonism and from there was increasingly drawn to Catholicism against the objections of her husband, eventually becoming a prominent Anglo-Catholic.

Her spiritual mentor from 1921 to 1924 was Baron Friedrich von Hügel, who was appreciative of her writing yet concerned with her focus on mysticism and who encouraged her to adopt a much more Christocentric view as opposed to the theistic and intellectual one she had previously held.

The enormous excitement in those days was mysteriously compounded of the psychic, the psychological, the occult, the mystical, the medieval, the advance of science, the apotheosis of art, the rediscovery of the feminine, the unashamedly sensuous, and the most ethereally "spiritual" (Armstrong, p. xiii–xiv).

This age of "the soul" was one of those periods when a sudden easing of social taboos brings on a great sense of personal emancipation and desire for an El Dorado despised by an older, more morose and insensitive generation.

She had found a little hole in the wall of appearances; and peeping through, had caught a glimpse of that seething pot of spiritual forces whence, now and then, a bubble rises to the surface of things.

Yet her novels give remarkable insight into what we may assume was her decision to avoid what St Augustine described as the temptation of fuga in solitudinem ("the flight into solitude"), but instead acquiescing to a loving, positive acceptance of this world.

(Cropper p. 46)Underhill tells how Suso's description of how the abstract truth (related to each soul's true nature and purpose), once remembered, contains the power of fulfilment and became the starting point of her own path.

(Ruysbroech)Where Underhill struck new ground was in her insistence that this state of union produced a glorious and fruitful creativeness, so that the mystic who attains this final perfectness is the most active doer – not the reclusive dreaming lover of God.

[15] Although he "seemed a nobody to those who did not know him", he was developing a strong spiritual life, "a penetrating intellect, a fearless heart, deep knowledge of human nature, remarkable powers of expression".

[53–54] An apostolic mystic [55] represents humanity in its quest to discern the Divine Reality, she writes, being like "the artist extending our universe, the pioneer cutting our path, the hunter winning food for our souls."

[57] His words about an ineffable Nature of God, "a dim silence, and a wild desert," may be suggestive, musical, she writes, "which enchant rather than inform the soul".

[61] "God as known by man" is the Absolute One who combines and resolves the contradictory natures of time and eternity, becoming and being; who is both transcendent and immanent, abstract and personal, work and rest, the unmoved mover and movement itself.

But beyond and above this eternal distinction lies "the superessential world, transcending all conditions, inaccessible to thought-- 'the measureless solitude of the Godhead, where God possesses Himself in joy.'

[67] Second, following the Son, "the power of knowing Divine things by intuitive comprehension: man's fragmentary share in the character of the Logos, or Wisdom of God."

[75, 76] "Love is a unifying power, manifested in motion itself, 'an outgoing attraction, which drags us out of ourselves and calls us to be melted and naughted in the Unity'; and all his deepest thoughts of it are expressed in terms of movement."

[78–79] At the end of her chapter IV, she discusses "certain key-words frequent in Ruysbroeck's works," e.g., "Fruition" [89], "Simple" [89–90], "Bareness" or "Nudity" [90], and "the great pair of opposites, fundamental to his thought, called in the Flemish vernacular wise and onwise."

[page 117] In the Enneads Plotinus presents the Divine as an unequal triune, in descending order: (a) the One, perfection, having nothing, seeking nothing, needing nothing, yet it overflows creatively, the source of being; [121] (b) the emitted Nous or Spirit, with intelligence, wisdom, poetic intuition, the "Father and Companion" of the soul; [121–122] and, (c) the emitted Soul or Life, the vital essence of the world, which aspires to communion with the Spirit above, while also directly engaged with the physical world beneath.

[125] Such might be the merely logical outcome for the metaphysician, yet Plotinus the seeker also presents this return to the Divine as a series of moral purgations and a shedding of irrational delusions, leading eventually to entry into the intuitively beautiful.

Her fiction was also influenced by the literary creed expounded by her close friend Arthur Machen, mainly his Hieroglyphics of 1902, summarised by his biographer: There are certain truths about the universe and its constitution – as distinct from the particular things in it that come before our observation – which cannot be grasped by human reason or expressed in precise words: but they can be apprehended by some people at least, in a semi-mystical experience, called ecstasy, and a work of art is great insofar as this experience is caught and expressed in it.

Incorporating the Holy Grail into their fiction (stimulated perhaps by their association with Arthur Waite and his affiliation with the Hermetic Order of the Golden Dawn), for Machen the Holy Grail was perhaps "the" hieroglyph, "the" crystallisation in one sacred emblem of all man's transcendental yearning, "the" gateway to vision and lasting appeasement of his discontents, while for her it was the center of atonement-linked meanings as she pointed out to Margaret Robinson in a letter responding to Robinson's criticism of Underhill's last novel: Don't marvel at your own temerity in criticising.

We have not fulfilled our destiny when we have sat down at a safe distance from it, purring like overfed cats, 'suffering is the ancient law of love' – and its highest pleasure into the bargain, oddly enough.

Also, she describes the fashionable creed of the time as "vitalism" and the term adequately sums up the prevailing worship of life in all its exuberance, variety and limitless possibility which pervaded pre-war culture and society.

The central doctrine of the Divine Fatherhood, and of the soul's power to become the Son of God, it is this raised to the nth degree of intensity ... and demonstrated with the exactitude of the mathematician, and the passion of a poet, which Ruysbroeck gives us ... the ninth and tenth chapters of The Sparkling Stone the high water mark of mystical literature.

He introduced her to the spiritual genius of India which she expressed enthusiastically in a letter: This is the first time I have had the privilege of being with one who is a Master in the things I care so much about but know so little of as yet: & I understand now something of what your writers mean when they insist on the necessity and value of the personal teacher and the fact that he gives something which the learner cannot get in any other way.

After converting to Anglicanism, and perhaps overwhelmed by her knowledge of the achievements of the mystics and their perilous heights, her ten-year friendship with Catholic philosopher and writer Baron Friedrich von Hügel turned into one of spiritual direction.

Charles Williams wrote in his introduction to her Letters: 'The equal swaying level of devotion and scepticism (related to the church) which is, for some souls, as much the Way as continuous simple faith is to others, was a distress to her... She wanted to be "sure."

But do not build your faith upon them; do not make them an end when they exist only to be a means...I am not sure that God does want a marked preponderance of this or that work or virtue in our life – that would feed still further your natural temperament, already too vehement.

More than any other person, she was responsible for introducing the forgotten authors of medieval and Catholic spirituality to a largely Protestant audience and the lives of eastern mystics to the English-speaking world.

As a frequent guest on radio, her 1936 work The Spiritual Life was especially influential as transcribed from a series of broadcasts given as a sequel to those by Dom Bernard Clements on the subject of prayer.