Author Leonard Levy, in his book Blasphemy, notes that "heresy" is not a Hebrew term. In fact, there is no equivalent for it in the Pre-Christian era. Heresy, a view not consistent with the church, depends entirely on the existence of an orthodox religion.
Both orthodoxy and heresy were foreign ideas to the earliest Christians, who lived with a variety of interpretations of the Gospels and church practices depending on locale and local teaching. All that ended in the late 4th century C.E. Several factors contributed to the change. In 313 C.E., after Constantine united the empire, Christianity became the state religion. Constantine saw a unified creed or dogma as essential to a unified empire, and, importantly, the lack of such a creed as a threat to power. This political and strategic goal dovetailed neatly with a growing crisis in Christian thought, which struggled with the need to define a new religion in such a way as to retain the unity of God without sacrificing the divinity of Christ. Early church theologians had to get the doctrine right, not only for the future of the church, but because the "right path" to salvation was at stake.
A series of ecumenical councils, starting in 325 at Nicea, set out to establish the consistent doctrine of the Church, a creed for all. As a result, for the first time, Christians began to persecute one another for differences of opinion and faith. If one were found to hold views inconsistent with the teaching of the church, or heresy, they could be charged with crimes against both the church and state. Levy writes:
Within 15 years of 380 (just prior to the Council of Constantinople), imperial edicts deprived all heretics and pagans of the right to worship, banned them from civil offices, exposed them to heavy fines, confiscation of property, banishment and in certain cases, death. By 435, there were sixty-six laws against Christian heretics plus many others against pagans. The purpose of persecution was to convert the heretics and heathen, thus establishing uniformity.
Note that this uniformity was not just a matter of church but also a matter of the state. A cohesive church served as the basis for a uniform, strong political state.
Writers on Unitarian, Universalist, and Unitarian Universalist history have long pointed to these early heretics as our "spiritual ancestors," meaning that though we hold little in common with them in terms of theological belief, we do share a sympathetic understanding of the process of discerning a different teaching of the right to holding that different point of view as valid.
In considering "orthodoxy," "heterodoxy," and "heresy," it's important to reflect not only on the winners and losers of the arguments, but how posterity has recorded the discussions. For example, all of Arius' writings were destroyed. We know about his views only from those who condemned them.