Indeed, St. Paul was the acknowledged source of inspiration for the greatest of all Gnostic teachers, Valentinus, who is said to have been a disciple of Theudas (or Theodas), a friend and student of Paul. It is no secret that there are numerous distinctly Gnostic elements in Paul's writings. He speaks of "hidden mysteries" and "secret wisdom" that can be told only to those belonging to an elite. What appealed most to Gnostics, including Valentinus, however, was that Paul became an apostle as the result of his own gnosis rather than by association with Jesus. Paul's account of being "caught up to the third heaven," "whether in the body or out of body I do not know," and there learning "things that cannot be told, which man may not utter" (2 Cor. 12.2-4) fully qualified him as the Gnostic apostle par excellence. He was thus an excellent source for the teachings and apostolic succession of Valentinus.
Concerning his lifelong interest in matters Gnostic, the Dutch scholar Gilles Quispel, a noted Gnostic expert and an associate of C. G. Jung, tells a remarkable story. During the dark years of World War II, when life and the world seemed lacking in hope and joy, Quispel turned to the study of Valentinus. The inspiration, comfort, and faith he derived from the writings of Valentinus were instrumental in turning him into a devoted and thoroughly sympathetic scholar of Gnosticism. Very likely Quispel's experience is not unique, and in fact many people in the contemporary world are finding the message of this greatest of all Gnostic teachers highly relevant and helpful.
G. R. S. Mead called Valentinus "the great unknown" of Gnosticism, and indeed there is little information regarding his life and personality. He was born in Africa, probably within the territory of the ancient city of Carthage, around or before 100 A.D. Educated in Alexandria, in his prime years he transferred his residence to Rome, where he achieved high prominence in the Christian community between the years 135 and 160. Tertullian writes that Valentinus was a candidate for the office of bishop of Rome and lost the election by a rather narrow margin. Tertullian, who himself joined the heresy of Montanism, alleges that Valentinus fell into apostasy around 175. There is evidence, however, that he was never universally condemned as a heretic in his lifetime and that he remained a respected member of the Christian community until his death. He was almost certainly a priest in the mainstream church and may even have been a bishop. Tertullian also stated that Valentinus was personally acquainted with Origen, and one may speculate with some justification that Valentinus's influence on this orthodox church father was considerable.
The overall character of Valentinus's contribution has been accurately summarized by Mead:

The Gnosis in his hands is trying to . . . embrace everything, even the most dogmatic formulation of the traditions of the Master. The great popular movement and its incomprehensibilities were recognized by Valentinus as an integral part of the mighty outpouring; he laboured to weave all together, external and internal, into one piece, devoted his life to the task, and doubtless only at his death perceived that for that age he was attempting the impossible. None but the very few could ever appreciate the ideal of the man, much less understand it. (Fragments of a Faith Forgotten 2)

Valentinus, the Gnostic who almost became Pope, was perhaps the only man who could have achieved positive recognition for the Gnostic approach to the message of Christ. If he had been elected Pope, his hermeneutic vision, combined with his superb sense of the mythical, might have fostered a general flowering of the gnosis within the very fabric of the Church of Rome, leading to an authoritative paradigm of Gnostic Christianity that could not easily have been exorcised for centuries, if at all. The fact that circumstances and the increasing flood tide of a regressive pseudo-orthodoxy caused his efforts to fail must be reckoned among the greatest tragedies of the history of Christianity.

Stephan A. Hoeller. Gnosticism. New Light on the Ancient Tradition of Inner Knowing. Chapter nine. Visionaries and prophets: the great masters of Gnosis. Valentinus, a Gnostic for all seasons.