First Council of Nicaea: when Christianity was deformed and Jesus named son of God.

[BrianSoddingBoru4]

And the heresy is what made them a cult. That’s very nearly the definition of a cult - a relatively small group of people with a heterodox religious opinion.

Boru

Heresy according to whom? emperor Constantine and his priests? 
This was a filtration of the Roman religious institution from any opposing voice ! the belief that Jesus is a mere prophet was "normal" and even "accepted by many" across the empire:

https://www.britannica.com/topic/Arianism

Arianism, in Christianity, the Christological (concerning the doctrine of Christ) position that Jesus, as the Son of God, was created by God. It was proposed early in the 4th century by the Alexandrian presbyter Arius and was popular throughout much of the Eastern and Western Roman empires, even after it was denounced as a heresy by the Council of Nicaea (325).

The real heresy is the act of Constantine and his priests.
Nobody saw Jesus as a son of God but the "heretic" Constantine and his priests; he doomed his empire and produced a mutant heathen.

He produced "a trinity".

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Read history better, please.

It’s definitional. You can’t reasonably argue that a heterodox religious position is anything BUT heresy. Bear it mind that declaring a belief to be heretical doesn’t establish which belief is correct, only that the minority position is seen as a threat to the majority. And the Arians were decidedly in the minority. As I’ve told you before, the belief you call ‘Nicene Christianity’ was the majority position of most Christians before the Council ever took place. If you have actual evidence to the contrary, I’ll be happy to consider it.

Boru