Christianity in the ante-Nicene period was the time in Christian history up to the First Council of Nicaea. The second and third centuries saw a sharp divorce of Christianity from its early roots. There was an explicit rejection of then-modern Judaism and Jewish culture by the end of the second century, with a growing body of adversus Judaeos literature. Fourth- and fifth-century Christianity experienced pressure from the government of the Roman Empire and developed strong episcopal and unifying structure. The ante-Nicene period was without such authority and was more diverse.

The Ante-Nicene period saw the rise of a great number of Christian sects, cults, and movements with strong unifying characteristics which were lacking in the apostolic period. They had different interpretations of the Bible, particularly regarding theological doctrines such as the divinity of Jesus and the nature of the Trinity. One variation was proto-orthodoxy which became the international Great Church and in this period was defended by the Apostolic Fathers. This was the tradition of Pauline Christianity, which placed importance on the death of Jesus as saving humanity, and described Jesus as God come to Earth. Another major school of thought was Gnostic Christianity, which placed importance on the wisdom of Jesus saving humanity, and described Jesus as a human who became divine through knowledge.

The Pauline epistles were circulating in collected form by the end of the 1st century. By the early 3rd century, there existed a set of Christian writings similar to the current New Testament, though there were still disputes over the canonicity of Hebrews, James, I Peter, I and II John, and Revelation.

There was no empire-wide persecution of Christians until the reign of Decius in the 3rd century. The Kingdom of Armenia became the first country in the world to establish Christianity as its state religion when, in an event traditionally dated to the year 301, Gregory the Illuminator convinced Tiridates III, the King of Armenia, to convert to Christianity.