Outside "historical Christian teaching"?... Well, Nicene is 300 years after Jesus died... so... actually Nicene is the novel idea and so-called heresies are the historical normative.
Erm, no. The reason why Arianism was so controversial was because, far from being "the historical normative", Arius' teachings were clearly contrary to mainstream Christian opinion c. AD 300. The Church hadn't formalised or systematised its beliefs on the Trinity before that point, but that's because Trinitarianism had hitherto been so uncontroversial that they didn't need to, not because the Council of Nicaea was making up some novel belief which nobody had held before.
Don't think Jesus ever spoke in Nicene nor spoke the creed; there's a difference between pure Christianity as the Jesus cult of Judaism with his direct sayings and Pauline Christianity as evolved and modified by Catholicism (Eastern or Roman).
There's no proper historical evidence that Paul marked a major break between "pure Christianity" and those nasty old Catholics. Such ideas belong in the realm of Dan Brown novels, not serious history.
When non-Christians dispute the monotheism and point to idolatry in Christianity; they are directly referencing not that the Father, Son, and Holy Ghost are separate, that's irrelevant, the fact that the three exist as part of a whole is a violation of the second commandment "you shall have no others before me" and the Shema "I am one",
The guy who propounded the idea that the Trinity consists of parts of God, Sabellius, was excommunicated for heresy in 220.
the crucifix and the cross as a violation of idolatry.
It would be, if Christians worshipped crucifixes; as they don't, it isn't.