Actually, unlike Trinitarianism, both Arian and
Sabellian or modalistic theology resulted in large measure from the influence of pagan
thought upon Christianity.  This fact is the affirmation of real historians quoted in context, rather than the creation of quotations slashed and burned from their contexts or from the assorted weirdos  cited by the Watchtower Society. “[The
system of] Sabellius . . . sprung out of Judaizing and Gnostic tendencies which
were indigenous to Egypt. . . . [A] pantheistic tendency [also characterizes]
Sabellianism as a whole. . . . Kindred ideas are also found in Pythagoreanism.”[1]  “[O]pposition to the Incarnate Word,
when he really appeared, seemed to have predisposed [modalists, here discussed under
the label of Monarchians] to accept a heathen philosophy, and to represent the
Logos as Philo did as the manifest God not personally distinct from the
concealed Deity.  This error found
its way into Christianity through the Gnostics, who were largely indepted to
the Platonic school of Alexandria. . . . Sabellianism [in part is] found even
in the later schools of gnostics, and the later Sabellianism approached to an
emanation theory. . . . The leading tenet of the Monarchians [modalists] thus
appears to have been introduced into Christianity principally through the
Alexandrian Jews and the Gnostics. 
It may also have been derived immediately from heathen philosophers. . .
. [T]he Monarchians who identified the Son with the Father and admitted at most
only a modal trinity, a threefold mode of revelation . . . proceeded, at least
in part, from pantheistic preconceptions, and approached the ground of Gnostic
docetism.”[2]  Modalism is a concept which mixes
Christianity and paganism.

Similarly, “Arius . . . was following . . . a
path inevitably traced for him by the Middle Platonist preconceptions he had
inherited,”[3] since “the
impact of Platonism reveals itself in . . . thoroughgoing subordinationism.”[4]  The Arian view of the incarnation of
Christ “took as its premis[e] [a] Platonic conception.”[5]

Examining the history of ancient Christianity,
one notes that no physical evidence exists of Arius, Sabellius, or the
disciples of either of these heretics affirming and disperaging Trinitarian
doctrine as derived from paganism, while testimony from ancient Christiandom
affirms that modalist and Unitarian heretics derived their ideas from
paganism.  The Trinitarian
Tertullian spoke strongly against the adoption of pagan philosophy, mentioning
that “Plato has been the caterer to all these heretics” and speaks of
“doctrines which the heretics borrow from Plato.”[6]  He writes, “Indeed heresies are
themselves instigated by [pagan] philosophy.”[7]  Specifically speaking against the
Unitarian heresy, Athanasius declared, “when the unsound nature of their
phrases had been exposed at that time, and they were henceforth open to the
charge of irreligion, that they proceeded to borrow of the Greeks [pagan
philosophy] . . . so unblushing are they in their irreligion, so obstinate in
their blasphemies against the Lord. . . . they are contentious, as elsewhere,
for unscriptural positions . . . [their language, namely, adopting the term
“Unoriginate” for God over “Father,” is] of the Greeks who know not the Son.”[8]  Ambrose wrote, “Let us now see how far
Arians and pagans do differ. . . . The pagans assert that their gods began to
exist once upon a time; the Arians lyingly declare that Christ began to exist
in the course of time. Have they not all dyed their impiety in the vats of philosophy?”[9]  The evidence from patristic writers
affirms that the doctrines of Arianism, Sabellianism, and other heresies were
influenced by paganism.  No extant
ancient writer affirms that the Trinity was borrowed from pagan
philosophy.  Who is more likely to
be correct on the development of Trinitarian theology—those who lived in the
first centuries of Christianity, or the wackos quoted by modern Arians and
Sabellians who lived a millenium and a half after the end of the ancient church
period?

Indeed, even the pagans testified that the Christians believed that the Lord Jesus Christ was God.  For example, the Roman official Pliny the Younger wrote to the the
Roman emperor Trajan c. A. D. 112: “I ask them if they are Christians. If they
admit this, I ask them the question again a second and third time, threatening
them with the death sentence if they persist. . . . But they declared that
their only crime or error was that they used to meet regularly before daybreak
on an appointed day, and to sing a hymn to Christ as to God, and to bind
themselves by an oath not to commit any crime, but to abstain from theft,
robbery, adultery or breach of trust, and not to deny a deposit when this was
required” (Pliny, Letters 10.96, in The
Letters of Pliny, 2 vols., trans. William
Melmoth. Cambridge: Harvard University Press, 1915, 2:403ff.).

Anti-Trinitiarian affirmations that the Trinity is derived from paganism are radically anti-historical–just as they are radically anti-Biblical.

-TDR