During the early centuries of the Church, various heresies arose that sought to change the Church’s theology and convert others to the heretics’ own beliefs. A prominent heresy called Donatism was started in the 3rd century. By that time many of the large-scale Roman persecutions had ended, but during some of the earlier tyranny, some Christains had temporarily denied their faith to be spared. Donatism taught that these people could never be readmitted to the Church. 

Although Donatism became a fairly influential heresy, it was nothing compared to Arianism, which at its height had almost as many followers as true Christianity. Arianism was most popular in the 4th century AD, and its doctrine taught that Christ was not equal to the Father and not of the same essence, the Father allows Christ certain divine attributes, and Christ is the agent of creation, but Christ himself is still a creature. Arians were still devoted to Christ, while saying that Christ is not God of His very essence, but that Christ grew to divinity through His own efforts. True Christian doctrine states that Christ is both fully God and fully man, and there is an equal trinity in one God. 

At the Council of Nicea (325), also known as the First Ecumenical Council, Arianism was declared a heresy, but still continued for years after this. Another extremely important event at the Council of Nicea was the establishment of the Nicene Creed, which is basically the Christian confession of faith. In 381 the First Council of Constantinople dealt further with Arianism, added to the Creed, and discussed the Holy Spirit. The Council of Chalcedon in 451 stated that Christ was “two natures, one person.”

Monasticism became a common practice in the early centuries of the Church and is still practiced in modern times. Two different forms of monasticism arose: Eremitic and Cenobitic. Eremitic is a life of absolute isolation as a hermit and Cenobitic is a life in a monastery with other monastics. Eventually most monastics became Cenobitic, and today there are few to little Eremitic monastics. Eremitic monasticism was much more common in the east of the empire while cenobitic was practiced in the west of the empire. Eremitic monasticism often involved penitential regiments, which is the action of discomfort or depriving oneself of food or water for long periods of time. This is also known as “dry martyrdom” because while one doesn’t actually die, they give themselves completely over to God. The practice of monasticism was widely used even after the fall of the Roman Empire and by the 14th century there were 37,000 monasteries.