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Arianism -

Arianism emerged from the principles taught by Arius, a presbyter and priest of Cyrenaic origin. Their beliefs closely resembled those of the early Church, with the exception of considering Jesus to be of lesser stature than God.

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Heresy -

Arius' idea was officially deemed heretical at the First Council of Nicea in 325 CE. Though a few Roman emperors supported Arius, with the establishment of Catholicism as the official religion, Arianism faded away.

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Adoptionism -

Although it traces its roots to the 2nd century, the advocacy of Adoptionism only gained momentum in the 8th century, with the efforts of a Spanish bishop named Elipandus. In essence, Adoptionists upheld the belief that Jesus was merely a man, lacking divine nature at birth.

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Adoptionism -

But Jesus received recognition for his innocence through baptism, and consequently became an adopted son of God. Over time, the movement was deemed heretical and the sect disbanded.

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Novatianism -

Novatians disagreed with the Catholic Church's belief that Christians who had succumbed to pressure from Rome and renounced their faith by public sacrifice should be readmitted into the Church.

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Novatianism -

The movement was initiated by Roman priest Novatian, who held a different opinion from Pope Cornelius and became an antipope. Eventually, the sect weakened and died out. 

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Docetism -

Docetism argued that Jesus Christ did not possess an actual human body, perceiving it as merely an illusion or semblance. This ideology stemmed from the belief that the human body was susceptible to corruption, thus contradicting Jesus' status as a mortal being.

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Docetism -

Docetism was one of the first Gnostic sects, dating back to the end of the first century. There's a criticism of their schism in the Bible, too. "This is how you can recognize the Spirit of God: Every spirit that acknowledges that Jesus Christ has come in the flesh is from God, but every spirit that does not acknowledge Jesus is not from God. This is the spirit of the antichrist, which you have heard is coming and even now is already in the world," (John 4:2-3).

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Donatism -

Caecilian became bishop of Carthage in the 4th century and received consecration from a bishop who was known to have surrendered scriptures during the Roman persecution.

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Donatism -

Majorinus of Carthage and the unforgiving opponents of the traditors stood against Caecilian. Majorinus made attempts to gain recognition for their faction from Constantine, but once St. Augustine assumed authority, the Donatist schism faced its demise.

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The Encratites -

The name of this group is derived from the Greek term meaning "self-control." Its origins date back to the 2nd century and revolve around an individual named Tatian.

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The Encratites -

The Encratites abstained from sexual activity and disapproved of marriage. They also refrained from eating meat and consuming alcohol. Theodosius the Great ultimately banned the sect, resulting in their decline and eventual disappearance.

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The Naassenes -

The Naassenes, an ancient Gnostic group, were established in the 2nd century and believed in James the Just, Jesus' brother.

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The Naassenes -

The Naassenes were known for their veneration of a serpent, as described by Christian theologian Hippolytus. They believed Mariamne, a follower of James the Just, conveyed their teachings.

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Catharism -

Catharism originated in 11th-century Western Europe, holding the belief that the spiritual world was created by God, while Satan was responsible for the material world. Salvation could only be achieved through a ritual called "consolamentum."

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Catharism -

The Cathari held the belief that the soul was an angel without gender, subjected to reincarnation as a result of Satan's condemnation. They abstained from consuming meat and maintained strict boundaries with individuals of the opposite sex. This movement flourished notably in southern France, where numerous Cathar castles can still be found.

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Bogomilism -

This group originates from Bulgaria and is relatively new, with a history that can be traced back to the 10th century. According to their beliefs, God had two sons named Satanael and Michael. Satanael rebelled against God, descended to Earth, and was responsible for the creation of the physical world and human bodies. On the other hand, Michael is considered the divine figure who created the human soul.

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Bogomilism -

Bogomils were opposed to the flaunting of riches in churches and embraced a modest way of living. They abstained from alcohol, sex, and meat. Bogomils were against the clergy, baptism, and physical symbols like crosses. The movement ceased to exist in the 16th century. Some remnants remain, like this grave marker in Bosnia and Herzegovina.

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The Cainites -

The group known as the Cainites derived their name from Cain, the son of Adam and Eve, who they believed had been unfairly punished by God. They held the belief that the world's creation was a malevolent act, designed to prevent humanity from reaching divine status.

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The Cainites -

The belief existed that Judas Iscariot did not betray Jesus, but simply followed his instructions. The Gospel of Judas, a gnostic text, purportedly contains conversations between Jesus and Judas for those interested in delving further. However, this sect's duration in the 2nd century was short-lived.

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Phibionites -

The Phibionites, also called Borborites, had peculiar rituals involving the consumption of menstrual blood and semen, considering them as the blood and body of Christ. These practices were part of their sexual sacramentalism.

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Simonians -

This group derived their teachings from Simon Magus (the Magician), who is regarded as a heretic and engaged in a well-known dispute with Paul the Apostle in Acts 8:9–24. They faced allegations of practicing magic and existed from the 2nd to the 4th century.

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Montanists -

Montanists, also known as the "New Prophecy," anticipated the arrival of modern Pentecostalism due to their focus on the Holy Spirit and speaking in tongues. They promoted the belief in Christ's imminent return, encouraging asceticism and martyrdom. However, they were later labeled heretics and faced persecution.

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Marcionites -

The Marcionites were a group of people who followed Marcion of Pontus. They disagreed with the belief that the Jewish God Yahweh was the father of Jesus. Furthermore, Marcion rejected all Jewish texts, which would later be known as the Old Testament. He firmly believed that Paul was the sole genuine apostle and promoted celibacy while prohibiting marriage. Similar to other Gnostics, he held the belief that Jesus was never a human being.

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Carpocratians -

Carpocrates, unlike mainstream Christians, advocated for licentiousness and embraced the concept of rebirth. The Secret Gospel of Mark, unearthed during the 20th century, sheds light on their ideology.

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Marcosians -

The Marcosians, under the guidance of their leader Marcus, followed the principles of numerology and letter theory. Influenced by the Pythagoreans, they associated letters with specific numbers. For example, the name "Jesus" corresponds to 888, which is derived by adding up the numerical values of all 24 letters in the Greek alphabet.

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Valentinians -

Valentinians adhered to the principles established by Valentinus, a Gnostic theologian. Valentinus held the belief in a Primal Being that possessed both masculine and feminine qualities. The Valentinians further elucidated their beliefs through the Gospel of Truth, a composition of their own.

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Basilideans -

The Basilideans were the followers of Basilides, an individual from Alexandria. They held dualistic and emanationist beliefs, which suggest that matter and spirit exist as distinct and conflicting forces. In their perspective, Jesus, being a divine being, did not possess a physical body and therefore could not have been crucified. The Basilideans believed that Jesus traded places (and appearances) with Simon of Cyrene, who was ultimately crucified instead of Jesus. 

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Ophites -

The Ophites derived their name from "snake," and their eucharist featured a snake. They believed that the serpent who tempted Eve was a heroic figure. This stemmed from their belief that God intended to keep Adam and Eve ignorant and under his control.

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Sethians -

The Sethians greatly admired Seth, who was the third son of Adam and Eve. They composed the Apocryphon, also known as the "Secret Book" of John. Within their writings, they mention an Unknown God, who is depicted with both male and female characteristics.

Sources: (Listverse) 

See also: The biggest unanswered questions in the Bible

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