Some of the earliest ideas that orthodox Christians found heretical dealt with the Trinity and the nature of Jesus and the Father. The Trinity basically is a belief that God is three beings having one essence: the Father, Son, and Holy Spirit, with Jesus being both human and divine. The Trinity is a Christian mystery — a matter of belief and faith rather than a rational conclusion. The orthodox early church believed that Christ died and that his death was a Paschal sacrifice that redeemed humanity by taking away the sins of the world. The church considered Christ's sacrifice as a gift from God, the Father, and at the same time a gift that the Son of God offered humanity by giving life to the Father through the Holy Spirit.

As Christianity spread and flourished, the orthodox fathers had to vigorously defend assaults of heresy, particularly against the Gnostics. Among the various communities of Gnostics could be found a variety of understandings and portrayals of Jesus. The Valentinians, for example, saw him as a savior figure while Gnostic believers in Seth viewed Jesus as the incarnation of Seth. The diversity of beliefs among Christians extended into other areas of belief as well.

As already mentioned, the role of women as church leaders, according to some modern scholars, was seen as a threat to the version of Christianity that was growing ever more powerful in the early second century. Women themselves were not mounting any kind of threat, but whatever gains they may have made began to erode. Still, some Gnostic sects allowed women to serve in the highest ecclesiastical roles. Their theology — with the Holy Spirit being female and women being light-and wisdom-bearers, the manifestation of the Divine in feminine form — carried over into the Gnostics' “real-world” treatment of women. Such views, however, were condemned by the later orthodox Christian church. With such ideas challenged and defeated, teachers of such ideas excommunicated, and writings eliminated, the Gnostics would have a tough time remaining a viable branch of anything. Even today, the church is ever-vigilant for ideas that are in error from its position. The following list sets forth ten heresies the proto-orthodox Christian fathers identified and attempted to eliminate.

• Adoptionism: Advocates of this heresy believed that God the Father adopted the human child Jesus (either at conception or during his baptism), and that Jesus then became divine. The tenth-century Bogomils were adoptionists who did not believe Mary was the mother of God nor Jesus part of a Trinity. Some say the French Cathars were adoptionists as well.

• Apollinarianism: Followers of Apollinarius espouse a belief that Jesus was neither human nor divine. They asserted that the eternal Logos “overshadowed” his mind but that he possessed a human body and soul.

• Arianism: Followers of Arius believed that Jesus Christ had been created for humanity's salvation. Jesus was not of the same substance as the Father, and, because he was created, was not eternal with the Father. Arianism spread to Africa, Italy, and Spain.

• Donatism: This mostly North African group followed Donatus the Great, who emphasized extreme asceticism, including martyrdom.

• Docetism: The Docetists believed that Christ was divine and that he only appeared to be human. The crucifixion of his physical body was only an illusion, since he was a divine spirit and could not therefore die.

• Gnosticism: One of the most prevalent and serious of the heresies faced by the early Church, continuing into the Middle Ages. Countless sects of Gnostics espoused diverse beliefs but a common belief involved gnosis as the means to salvation.

• Monophysitism: The Monophysites held the view that Jesus Christ was two separate beings — human and the divine, eternal Logos fused together into one body during his time on earth.

• Pelagianism: This sect did not arise until around the fifth century when Pelagius, an Irish priest, called for the end of baptism, asserting that people could save themselves through individual effort. Followers of this sect did not believe that Adam and Eve passed down original sin.

• Sabellianism: The Sabellianists or followers of Sabellius believed that Jesus Christ and the Holy Spirit along with God the Father were just three facets of One Being. They thought that Jesus Christ was not human, but fully divine.

• Manichaeanism: Early orthodox fathers considered this a serious heresy because of the way it represented Christian ideas blended with Mithraism, neo-Platonism, Gnosticism, and possibly even Buddhism. Christian Saint Augustine flirted with this doctrine before rejecting it.