From the earliest days of Christianity, heresy was a serious matter that threatened the cohesiveness of the Church and was believed to put souls at risk. It was therefore fought with the vehemence with which modern doctors fight disease.
When a heretic was discovered, Church officials attempted to persuade him through logic and reason that his viewpoint was wrong and that he should return to the fold. Should they succeed, the heretic would abjure and perform a penance to be accepted back into the Church. Should they fail, the heretic would be excommunicated. If a heretic abjured and then recanted his abjuration, he would be handed over to the secular authorities for punishment, usually execution by burning at the stake.
Throughout the Middle Ages, the Church dealt with a variety of heresies, and over time, its definition of heresy evolved to the point where mere dissent qualified. More severe methods began to be implemented in uncovering heretics and extracting confessions. The judicial tool of inquisition was created to root out and destroy heresy in all its forms, and unrepentant heretics were routinely handed over to the secular authorities for punishment, often execution.
After the Reformation, some Protestant groups saw their own interpretation of doctrine as the only true way, and viewed all others as heretical. However, attitudes changed as a variety of Protestant sects established themselves as reasonable authorities. While the Catholic Church still denounces heresy, tolerance is much more prevalent today.
Arianism was just one of many early heresies denounced by the Christian Church.
Joan of Arc was burned at the stake as a heretic and a witch.
Protestant propagandists tended to paint early heretics as martyrs and heroes to the Protestant cause, whatever their views had actually been.
Cathars were a heretical group that arose in Europe in the 11th century.