Philo was a prominent Jewish intellectual from Alexandria, Egypt. Philo, who was a Hellenistic Jew, was active in the first century and employed an allegorical method of interpretation. He allegorized many Old Testament passages which the Greek mind found distasteful. His method heavily influenced many of the Church Fathers, most notably Origen of Alexandria.
Marcion was (formerly) a man of means who came to Rome in the mid-first century AD. He nearly became bishop of the city but eventually was opposed by the orthodox for teaching his own brand of Gnosticism. His version did not include as nearly as many fanciful elements as some of the other strands and may have been seen as more “plausible”. This, plus his apparent likability and giftedness as a teacher, made him quite dangerous. His heretical views even led him to create his own New Testament canon, which included a mutilated version of Luke and Paul’s writings only; save the Pastoral Epistles.
Arius was a charismatic false teacher whose theology can be summed up with the phrase “there was a time when he was not” (referring to the Son). Arius put his heresy into jingles and soon Arianism was popular. Arius taught that the Son was a created being and could be called “god” in name only but was inferior to the Father. His heresy was first noticed by Alexander of Alexandria and would later be combatted (and defeated) by Athanasius.
Adoptionism is a Christological heresy which teaches that at Jesus’ baptism, God adopted the man Jesus. At that moment, the Christ spirit came into the man Jesus and then he became a unique man with a special Christ spirit.
Modalism was a heresy relating to the Trinity, which Modalists denied. They wanted to preserve the oneness of God and in so doing smashed down the three persons of the Godhead into three different modes of being, almost as if the one God wears three different masks depending on his role.
Sabellianism was essentially a more advanced version of modalism. Sabellius sought to answer some of the questions in regards to this doctrine and did so with more theological rigor and philosophical nuance than those that came before him. He still denied that there were three co-eternal and simultaneous persons within the Godhead.
Docetism is a Christological heresy that arose as early as the late first century (I John may allude to it). Ignatius of Antioch certainly fought against it in the first decade of the second century. Docetism taught that the Christ only seemed to possess flesh (from the Greek word dokeo, “to seem”). In reality, the Christ was more of a phantasm who appeared to be a man. The Docetic Gospel of Peter taught that at the crucifixion the Christ spirit abandoned the man Jesus, when it put these words on the lips of Jesus, “My power, my power, why have you left me?”
Gnosticism was a syncretic leech; a philosophy and worldview which utilized Christian symbols to tell its own stories. This included a cosmology in which Yahweh was an inferior and ignorant demigod who created matter out of his own stupidity and arrogance and a spiritual hierarchy in which only esoteric knowledge could enable the enlightened to prevail. This anti-Jewish heresy became a heretical cancer in the early church, producing many forged writings and schisms along the way before it was stamped out by apologists such as Irenaeus.
The Council of Nicea
This important ecumenical council took place in what is now Turkey in the summer of 325 AD. Arianism threatened to rip apart the unity of the church and Constantine, an emperor who wanted the church to be unified, called a council to fix the problem. Bishops came from all over and together condemned Arianism and declared that the Son was of the same substance as the Father. This means He was not only eternal but equal to the Father and was not a created being.
The Twenty Canons
Twenty canons (or rulings) came forth out of Nicea. Many dealt with issues of church discipline, for example, what to do with the lapsed. Others dealt with issues of ecclesiastical jurisdiction and church government. Some dealt with the date of Easter.