During the 4th and 5th centuries, heresies and controversies started popping up in the Church. Donatism was an early deviation that was started by the Bishop of Carthage, who stated that the validity of the sacrament was based on the priest giving it, while Montanism proclaimed that there would be new prophets to warn of the Second Coming of Christ. One of the most sinister heresies was Arianism. Starting around the year 300, the Alexandrian Presbyter Arius preached that Christ was not equal to the Father and that the Son was not fully God. Many emperors and bishops agreed with this doctrine and persecuted people practicing the true faith. 

Arianism was condemned in 325 at the First Council of Nicea, which also made advancements in Church governance. The Council of Constantinople in 381 dealt further with Arianism and made additions to the Nicene Creed. Another influential subversion was monophysitism, the belief that Christ only had one nature, which was divine. It was struck down at the Council of Chalcedon in 451. 

There are two main ways to practice Christian monasticism: Eremitic – monks (sometimes called hermits) live in isolation in a secluded place like a forest or desert; and Cenobitic – a group of monks who live and pray together in a monastery. St. Anthony of Egypt (250-335) left his monastery, went to be a hermit in the desert of Egypt, and asked that he be left in solitude. However, 300 people followed him and his example and also became hermits. 

By the year 300, women also started becoming monastics and are now known as nuns. St. Benedict made many of the rules followed today in Western monasteries. The main center of Western monasticism is Monte Casino and the East’s is Mount Athos. Monks also invented many agricultural techniques, wrote a large amount of books and writings, and started copying manuscripts that were used in Medieval Europe. 

Many early Christian apologists thought that the ancient Greek philosophers had a root in the Christian faith, like St. Justin Martyr. Originally a Platonist, he was converted to Christianity and preached extensively to Jews and Pagans, eventually being beheaded for his faith. 

The early heresies of the Church such as Arianism, Monophysites, and Origensim, were superseded by later problems like Iconoclasm and Muslim empires that invaded christian lands and would later cause war between the two religions in the Middle Ages.