Valentinus produced a variety of writings, but only fragments survive, not enough to reconstruct his system except in broad outline. His doctrine is known to us only in the developed and modified form given to it by his disciples. He taught that only some Christians, his own followers, received the gnosis (knowledge) that allowed them to return to the divine pleroma, while other Christians would attain a lesser form of salvation, while the rest of humankind was doomed to eternal perdition.
Valentinus had a large following. It later divided into an Eastern and a Western or Italian branch. The Marcosians belonged to the Western branch.
Valentinus taught first in Alexandria and went to Rome about 136 AD, during the pontificate of Pope Hyginus, and remained until the pontificate of Pope Anicetus. In Adversus Valentinianos, iv, Tertullian says:
According to a later tradition, he withdrew to Cyprus, where he continued to teach and draw adherents. He died probably about 160 or 161 AD.
While Valentinus was alive he made many disciples, and his system was the most widely diffused of all the forms of Gnosticism, although, as Tertullian remarked, it developed into several different versions, not all of which acknowledged their dependence on him ("they affect to disavow their name"). Among the more prominent disciples of Valentinus, who, however, did not slavishly follow their master in all his views, were Bardasanes, invariably linked to Valentinus in later references, as well as Heracleon, Ptolemy and Marcus. Many of the writings of these Gnostics, and a large number of excerpts from the writings of Valentinus, existed only in quotes displayed by their orthodox detractors, until 1945, when the cache of writings at Nag Hammadi revealed a Coptic version of the Gospel of Truth, which is the title of a text that, according to Irenaeus, was the same as the Gospel of Valentinus mentioned by Tertullian in his Adversus Valentinianos.
The Christian heresiologists also wrote details about the life of Valentinus, often scurrilous. As mentioned above, Tertullian claimed that Valentinus was a candidate for bishop, after which he turned to heresy in a fit of pique. Epiphanius wrote that Valentinus gave up the true faith after he had suffered a shipwreck in Cyprus and became insane. These descriptions can be reconciled, and are not impossible; but few scholars cite these accounts as other than rhetorical insults.
Valentinian literature described the Primal Being or Bythos as the beginning of all things who, after ages of silence and contemplation, gave rise to other beings by a process of emanation. The first series of beings, the aeons, were thirty in number, representing fifteen syzygies or pairs sexually complementary. Through the error of Sophia, one of the lowest aeons, and the ignorance of Sakla, the lower world with its subjection to matter is brought into existence. Man, the highest being in the lower world, participates in both the psychic and the hylic (material) nature, and the work of redemption consists in freeing the higher, the spiritual, from its servitude to the lower. This was the word and mission of Christ and the Holy Spirit. Valentinus' Christology may have posited the existence of three redeeming beings, but Jesus while on Earth had a supernatural body which, for instance, "did not experience corruption" by defecating (Clement, Stromateis 3.59.3 translated B. Layton p. 239); there is also no mention of 1 Peter's nor any other account of Jesus's suffering in any Valentinian text. The Valentinian system was comprehensive, and was worked out to cover all phases of thought and action.
Valentinus was among the early Christians who attempted to align Christianity with Platonism, drawing dualist conceptions from the Platonic world of ideal forms (pleroma) and the lower world of phenomena (kenoma). Of the mid-2nd century thinkers and preachers who were declared heretical by Irenaeus and later mainstream Christians, only Marcion is as outstanding as a personality. The contemporary orthodox counter to Valentinus was Justin Martyr.
In the fourth-century, Marcellus of Ancyra declared that the idea of the Godhead existing as three hypostases (hidden spiritual realities) came from Plato through the teachings of Valentinus. Valentinus is quoted as teaching that God is three and three prosopa (persons) called the Father, the Son and the Holy Spirit:
Since Valentinus had used the term hypostases, his name came up in the Arian disputes in the fourth century. Marcellus of Ancyra, who was a staunch opponent of Arianism but also denounced the belief in God existing in three hypostases as heretical (and was later condemned for his views), attacked his opponents (On the Holy Church, 9) by linking them to Valentinus:

It should be noted that Nag Hammadi library Sethian text such as Trimorphic Protennoia identify Gnosticism as professing Father, Son and feminine spirit Sophia or as Professor John D Turner denotes, God the Father, Sophia the Mother, and Logos the Son.
"Valentinians" is the name for the school of gnostic philosophy tracing back to Valentinus. It was one of the major gnostic movements, having widespread following throughout the Roman Empire and provoking voluminous writings by Christian heresiologists. Notable Valentinians included Heracleon, Ptolemy, Florinus, and Axionicus.
Later, Epiphanius of Salamis discussed and dismissed him (Haer., XXXI). As with all the non-traditional early Christian writers, Valentinus has been known largely through quotations in the works of his detractors, though an Alexandrian follower also preserved some fragmentary sections as extended quotes. A Valentinian teacher Ptolemy refers to "apostolic tradition which we too have received by succession" in his Letter to Flora. Ptolemy is known only for this letter to a wealthy gnostic lady named Flora, a letter itself only known by its full inclusion in Epiphanius' Panarion; it relates the gnostic view of the Law of Moses, and the situation of the Demiurge relative to this law. The possibility should not be ignored that the letter was composed by Epiphanius, in the manner of composed speeches that ancient historians put into the mouths of their protagonists, as a succinct way to sum up.