Tozer and The Catholic Mystics: While there is little question that Tozer was a man who was not afraid to ‘tell it like it is’ (a trait that is sadly wanting in the church today) and many modern day believers would do well to heed some of his uncompromising words. However one has to weight this against the fact that his purpose in life was to know God personally, and to this end he often references Catholic mystics, most notably in his
book ‘The Pursuit of God’, which along with ‘The Knowledge of the Holy’ is considered a spiritual classic.
This is what Tozer says in the Introduction to ‘The Pursuit of God’
This book is a modest attempt to aid God's hungry children so to find Him. Nothing here is new except in the sense that it is a discovery which
my own heart has made of spiritual realities most delightful and wonderful to me. Others before me have gone much farther into these holy mysteries than I have done, but if my fire is not large it is yet real, and there may be those who can light their candle at its flame. [Excerpt from the Introduction to ‘The Pursuit of God’. 1948]
When all is said and done, the Catholic mystics can not be considered
reliable guides in the spiritual arena. Here is a short introduction to the mystics that Tozer references, since it is very likely that many of Tozer’s readers know little or nothing about them.
Augustine, Bernard of Cluny (Clairvaux), St. Francis etc.
Bernard of Clairvaux was a French abbot and the primary builder of the reforming Cistercian order. As stated by the Catholic Encyclopedia, he
"...had a special devotion to the Blessed Virgin, and there is no one who speaks more sublimely of the Queen of Heaven".  Homilies in Praise of the Blessed Virgin Mary is even now available on Amazon.com. It is further claimed that St. Bernard played the leading role in the development of the virgin cult. It was only with the rise of emotional Christianity in the 11th century that she became the prime intercessor for humanity.
“In our knowledge of divine things three degrees may be distinguished: the knowledge furnished by reason, by faith and by spiritual experience respectively…”
“…The immanence of God in His universe makes possible the enjoyment of the “real Presence” by the saints of God in heaven and on earth simultaneously...
“I suggest also that we try to acquaint ourselves as far as
possible with the good and saintly souls who lived before our times... Augustine, for instance, would bring to us a sense of the overwhelming majesty of God... Bernard of Cluny would sing to us of 'Jerusalem the Golden' and the peace of an eternal sabbath day until the miserable pleasures of this world become intolerable;
"Richard Rolle would show us how to escape from “the abundance of riches, the flattering of women and the fairness of youth,” that we
may go on to know God with an intimacy that will become in our hearts ‘heat, fragrance and song’;
"Tersteegen would whisper to us of the ‘hidden love of God’ and the awful Presence until our hearts would become ‘still before Him’ and ‘prostrate inwardly adore Him’; before our eyes the sweet St. Francis would throw his arms of love around sun and moon, trees and rain, bird and beast, and thank God for them all in a pure rapture of spiritual devotion…”
[Tozer. Excerpt from Man the Dwelling Place of God. Chapter 12. Three Degrees of Religious Knowledge]
Baron von Hügel: (1852–1925)
was an influential Austrian Roman Catholic layman, often mentioned as one of the most influential Catholic thinkers of his day. According to a Gifford Lectures page
“Von Hugel was also deeply affected by liberal biblical criticism, which called into question the historicity of much
of the Bible, and he sought to overcome this by appealing to the sense of ‘transcendence’ which he considered basic to the human soul. If the soul is truly open to experience and appropriately guided, Von Hugel thought, it realizes that there are higher, deeper, more mysterious realities beyond the empirical, which are incapable of natural explanation. These constitute true religion.
Von Hugel became a student of this ‘experience’, especially of the phenomenon
of mysticism, in which the soul is drawn into a sense of complete union with God going far beyond any ability to express or formulate in words or images. Von Hugel’s chief work on religion was a study of a somewhat obscure medieval mystic, St. Catherine of Genoa”. 
All is of God, for as von Hugel teaches, God is always previous. [Tozer. Excerpt from Pursuit of God Chapter 1: Following hard after God]
Eastern mysticism has penetrated every area of Western society and much of the church. It may come as a It comes as a great surprise to many that the largest missionary organization in the world is not Christian but Hindu - India’s Vishva Hindu Parishad (VHP). This invasion from the East has spilled over into the church. Centering prayer, now a common practice in much of the church originated in St. Joseph’s Abbey, a Trappist monastery in Spencer, Massachusetts.
During the twenty years (1961–1981) when Thomas Keating was abbot, St. Joseph’s held dialogues with Buddhist and Hindu representatives, and a Zen master gave a week-long retreat to the monks.
See The Influence of Eastern Mysticism and
Contemplative Prayer - The Pioneers (Thomas Keating, Thomas Merton, Henri Nouwen and Brennan Manning) Note this is the concluding chapter of the article Contemplative Prayer.
"That God is everything and man nothing is a basic tenet of Christian
faith and devotion; and here the teachings of Christianity coincide with those of the more advanced and philosophical religions of the East.” [Tozer. Excerpt from Knowledge of the Holy, Chapter 5]
Tozer considered Augustine a “great saint”, which he stated in Removing the Veil, chapter 3 of The Pursuit of God. [See The Sins of Augustine]. Also...
"The experiential heart-theology of a grand army of fragrant saints is rejected in favor of a smug interpretation of Scripture which would certainly have sounded strange to an Augustine..." [Tozer. Excerpt from The Pursuit of God, Chapter 1: Following hard after God]
"Among the famous sayings of the Church fathers none is better known than Augustine's... 'Thou hast formed us for Thyself, and our hearts are restless till they find rest in Thee.’ The great saint states here in few words the origin and interior history of the human race...” [Tozer. Excerpt from The Pursuit of God, Chapter 3: Removing the Veil]
St. Francis, Luther, Thomas a Kempis etc.
The Imitation of Christ by Thomas a Kempis is widely considered one of the greatest manuals of devotion in pre-reformation Catholic Christianity. The Jesuits give it an official place among their "exercises". Of him Tozer said...
"Pick at random a score of great saints whose lives and testimonies are widely known. Let them be Bible characters or well known Christians of post-Biblical times... how unlike each other were John and Paul,
St. Francis and Luther, Finney and Thomas a Kempis... "Yet they all walked, each in his day, upon a high road of spiritual living
far above the common way... I venture to suggest that the one vital quality which they had in common was spiritual receptivity..." [Tozer. Excerpt from Pursuit of God, Chapter 5: The Universal Presence]
Nicholas of Cusa
Nicholas of Cusa, a German cardinal of the Catholic
Church was noted for his deeply mystical writings about Christianity, particularly on the possibility of knowing God with the divine human mind — not possible through mere human means. Shortly after the Fall of Constantinople in 1453, Nicholas wrote De pace fidei (the Peace of Faith). This visionary work imagined a summit meeting in Heaven of representatives of all nations and religions. Islam and the Hussite movement in Bohemia are represented. The conference agrees that there can be una
religio in varietate rituum, a single faith manifested in different rites, as manifested in the eastern and western rites of the Catholic Church. Tozer’s words...
"When all my endeavour is turned toward Thee because all Thy endeavour is turned toward me; when I look unto Thee alone with all my attention, nor ever turn aside the eyes of my mind, because Thou dost enfold me with Thy constant regard...' So wrote Nicholas of Cusa four hundred years ago.
"I should like to say more about this old man of God. He is not much known today anywhere among Christian believers, and among current Fundamentalists he is known not at all. I feel that we could gain much from a little acquaintance with men of his spiritual flavor and the school of Christian thought which they represent...
"Nicholas was a true follower of Christ, a lover of the Lord, radiant and shining in his
devotion to the Person of Jesus. His theology was orthodox, but fragrant and sweet... says Nicholas... 'With Thee, to behold is to give life; ’tis unceasingly to impart sweetest love of Thee; ’tis to inflame me to love of Thee by love's imparting, and to feed me by inflaming, and by feeding to kindle my yearning, and by kindling to make me drink of the dew of gladness, and by drinking to infuse in me a fountain of life, and by infusing to make it increase and endure.’ (Quoted from Nicholas of
Cusa, The Vision of God)
"When the habit of inwardly gazing Godward becomes fixed within us we shall be ushered onto a new level of spiritual life more in keeping with the promises of God and the mood of the New Testament. The Triune God will be our dwelling place even while our feet walk the low road of simple duty here among men. We will have found life’s summum bonum indeed..." [Tozer. Excerpt from Pursuit of God, Chapter 7: The Gaze of the Soul]