Arianism was/is a Christological doctrine which held/holds that the Son is distinct from and subordinate to God the Father: the Son is not 'of God'.
It was an interpretation of Jesus's divinity and relationship to God the Father that was opposed by others. Besides the issues of Jesus' relationship with the Father and his essence, there was a chronological aspect. Arius is said to have stated: "If the Father begat the Son, then he who was begotten had a beginning in existence, and from this it follows there was a time when the Son was not."
(Arianism (Ἀρειανισμός, Areianismós) and Arian were not descriptions that followers of Arian (c. AD 256–336, a Christian presbyter from Alexandria, Egypt), used of themselves ie. they were designations applied by others.
Controversy over Arianism arose in the late 3rd century and persisted throughout most of the 4th century. It involved most church members—from simple believers, priests, and monks to bishops, emperors, and members of Rome's imperial family. Two Roman emperors, Constantius II [r. 337-61] and Valens [r. 364-78], became Arians or Semi-Arians, as did prominent Gothic, Vandal, and Lombard warlords both before and after the fall of the Western Roman Empire [~476 or before]. Such a deep controversy within the early Church during this period of its development could not have materialized without significant historical influences providing a basis for the Arian doctrines.
https://en.wikipedia.org/wiki/Arianism#Origin
A corollary of this doctrine was firming of the different Nicene doctrine cemented at the Council of Nicea of describing Jesus (God the Son) as "same in being" or "same in essence" with God the Father (ὁμοούσιον τῷ Πατρί)
- homoousion / ὁμοούσιον = 'same in being, same in essence'
The term ὁμοούσιος had, however, been used before its adoption by the First Council of Nicaea: some so-called Gnostics are said to have first used ὁμοούσιος. There is apparently no trace at all of its existence beforehand. Oμοούσιος was/is used with the following meanings:
- Identity of substance between generator and generated.
- Identity of substance between things generated of the same substance.
- Identity of substance between the partners of a syzygy.
Basilides is said to have been the first known Gnostic thinker to use ὁμοούσιος (in the first half of the 2nd century AD) to speak of a threefold sonship consubstantial with the god who is not. The Valentinian Gnostic Ptolemy says in his Letter to Flora that it is the nature of the good God to beget and bring forth only beings similar to, and consubstantial with, himself.
Valentinus's name came up in the Arian disputes in the fourth century when Marcellus of Ancyra, a staunch opponent of Arianism, denounced the belief in God existing in three hypostases as heretical. Marcellus, who believed Father and Son to be one and the same, attacked his opponents by attempting to link them to Valentinus ... 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, who is quoted as teaching that God is three hypostases and three prosopa (persons) called the Father, the Son, and the Holy Spirit:
While this accusation is often [referenced or cited] in stating Valentinus believed in 'a Triune Godhead,' there is in fact no corroborating evidence that Valentinus ever taught these things. Irenaeus makes no mention of this in any of his five books against heresies, even though he deals with Valentinianism extensively in them. Rather, he indicates that Valentinus believed in the pre-existent Aeon known as Proarche, Propator, and Bythus who existed alongside Ennœa, and they together begot Monogenes and Aletheia: and these constituted the first-begotten Pythagorean Tetrad, from whom thirty Aeons were produced. Likewise, in the work cited by Marcellus, the three natures are said to have been the three natures of man, concerning which Irenaeus writes:Now with the heresy of the Ariomaniacs, which has corrupted the Church of God... These then teach three hypostases, just as Valentinus the heresiarch first invented in the book entitled by him 'On the Three Natures'. For he was the first to invent three hypostases and three persons of the Father, Son and Holy Spirit, and he is discovered to have filched this from Hermes and Plato.
According to Eusebius, Marcellus had a habit of mercilessly launching unsubstantiated attacks against his opponents, even those who had done him no wrong."They conceive, then, of three kinds of men, spiritual, material, and animal, represented by Cain, Abel, and Seth. These three natures are no longer found in one person, but constitute various kinds [of men]. The material goes, as a matter of course, into corruption." [Against Heresies, 1.7.5]
https://en.wikipedia.org/wiki/Valentinu ... c)#Trinity
The previous accounts notwithstanding - to show other pre-Nicene doctrines such as Valentinianism, at least, were invoked in doctrinal disputes -
Did Marcionism Feature in the Heretical Disputes of the Fourth Century?
I can't find any references to it being involved