Because they were resolved more than a thousand years ago, many of the heresies that plagued the early Church are unfamiliar to modern Catholics; often, they had to do with the struggle to understand the true nature of Christ. Arianism, for instance, which taught that Jesus, although the Savior, was not equal to God but merely His highest creation, was one of the most pernicious and took two ecumenical councils, one at Nicea and another at Constantinople, to finally refute.
Another heresy, perhaps less well known, was Monothelitism. This teaching, championed by the Byzantine Emperor himself, maintained that while on earth Jesus, rather than having a human and divine will as well as a human and divine nature, had only one will – a divine one. Although this, too, was finally refuted at an ecumenical council in 680-681, it was not before it was vigorously opposed by a pope, Martin I. Refusing to back down before the Emperor would cost Martin his life, and he thus became the last pontiff to be venerated as a martyr in the Church.
This was in an age when secular rulers – kings and emperors – saw themselves as people who could, and did, make ecclesiastical and theological decisions that should really have fallen within the purview of the Church. When Martin I, for instance, became pope, he did so without the confirmation and consent of the Byzantine Emperor, Constans II. That act alone would have set him on a collision course with royal authority, but when he went one step further and censured the documents that Constans had promulgated containing the heresy of Monothelitism, his fate was virtually sealed.
After trying unsuccessfully to turn the bishops and the faithful against Martin, Constans then decided to have the pope assassinated. This too failed and, enraged by the continued challenge to his authority, the emperor finally had the pope arrested and dragged in chains from Rome to Constantinople. Convicted of treason but narrowly avoiding execution, the elderly Martin none-the-less died in exile in the year 655 from the torture and ill treatment he suffered at the hands of the emperor.
Martin I would be vindicated 25 years later. When the Third Council of Constantinople was convened, the monothelitist heresy was decisively condemned and the teaching that Jesus possessed a perfectly united divine and human nature was upheld. Martin's feast day is celebrated on April 13.