There were many, just as there are today. But I can answer that question with four examples:
Valentinus
(2nd century A.D.)
From papal candidate to leading Gnostic
A brilliant theologian who taught in Alexandria, Egypt (the Oxford of his day), Valentinus moved to Rome in about A.D. 136 and quickly became a candidate for pope, then known as bishop of Rome. Not only was he not elected, he was excommunicated when he later emerged as leader of a heresy known as Gnosticism, which taught that only a select few receive gnosis ("knowledge" in Greek) from God about how to find salvation.
With this conviction, Valentinus proceeded to reinterpret the Bible—misinterpret, charged critics such as Irenaeus and Tertullian. For Valentinus, the most important lessons of Scripture came not from the obvious meaning but from the symbolism beneath the words. This method of biblical interpretation, called allegory, allowed Valentinus to create elaborate stories and teachings that blurred the lines between Christianity, mysticism, philosophy, and Judaism.
To the Genesis sketch of Creation, for example, Valentinus added a number of details. Throughout the ages, according to Valentinus, God produced 15 spiritual couples who personified divine characteristics such as goodness and truth. One being, Sophia (Greek for "wisdom"), rejected her partner because her only passion was to know everything about God. By herself she conceived and gave birth to a deformed child, whom she named Ialdabaoth (probably meaning "Child of Chaos"). Out of the elements of creation, her son (the diety portrayed in the Old Testament) produced the dark world of humanity and infused it with numbness toward God. Jesus, God's great revelator, came to awaken people to the "deep things of God."
For Valentinus and other Gnostics, there was no mixing of the spiritual world with the physical. Thus they rejected the incarnation, crucifixion, and bodily resurrection of Jesus.
Valentinianism endured merciless polemics by the church fathers for the first few centuries A.D. then faded into oblivion—until 1945.
Until then, all we knew of Valentinus came from his critics. But among the 52 documents recovered from the ruins of what was perhaps a Gnostic monastery near Nag Hammadi, Egypt, is a book written by Valentinus or his followers. Called The Gospel of Truth, it reads like a sermon and draws on the Gospels and the writings of Paul.
Novatian
(c. 200-258)
Fought for a pure church a little too hard
It was the spring of 251, and the Roman bishop was dead—martyred by Romans in a new wave of persecution. But raiders from the north were temporarily diverting the empire's attention, so Christians were breathing a sigh of relief. Two issues immediately confronted church leaders: (1) Who should they elect as the new bishop of Rome? (2) What should they do about "lapsed Christians," those who renounced their faith during persecution?
Novatian was the leading churchman in Rome, a brilliant theologian, and the obvious choice for pope. But he wasn't elected, perhaps because of his unpopular, hard-line position about the lapsed. He said they could never be readmitted to the church, and he invoked the words of Jesus: "Whoever denies me before men, I also will deny before my Father who is in heaven."
Cyprian, bishop of Carthage, a major North African city, did not agree. He called Novatian "a foe to mercy, a destroyer of repentance." The influential African bishop supported Cornelius, who was elected pope. Cornelius believed that the lapsed could be reinstated to the church by repenting and doing penance based on the seriousness of the offense. Christians who had offered sacrifices on Roman altars drew the stiffest penance.
