Nicene Christianity refers to Christian doctrinal traditions that adhere to the Nicene Creed, which was originally formulated at the First Council of Nicaea in 325 AD and finished at the First Council of Constantinople in AD 381. It is much more commonly referred to as mainstream Christianity.
The main rival doctrine of Nicene Christianity at the time was Arian Christianity, which ceased to exist during the 7th century AD with the conversion of the Gothic kingdoms to Nicene Christianity. The main points of dissent centered on Christology. Nicene Christianity considers Christ to be divine and co-eternal with God the Father, while Arian Christianity considered Christ to be the first created being, and inferior to God the Father. Other non-Nicene currents have been considered heresies since the early medieval period.
Present-day mainstream Christian Churches, including all of the Catholic, Eastern Orthodox, Oriental Orthodox, Assyrian, Lutheran and Anglican churches, together with most Protestant denominations, adhere to the Nicene Creed and are thus examples of Nicene Christianity.
Chalcedonian Christianity is a large subset of Nicene Christianity. In addition to subscribing to the Nicene Creed, Chalcedonian Christians also subscribe to the decisions of the First Council of Ephesus in 431 AD and of the Council of Chalcedon in 451 AD. The great majority of Nicene Christians are also Chalcedonian Christians. However, some portions of Eastern Christianity, such as the Oriental Orthodox Churches, adhere to the Nicene Creed, but not the Chalcedonian Definition, and are therefore part of Nicene Christianity but non-Chalcedonian.
Examples of non-Nicene Christianity today include the various non-trinitarian groups, such as the Unitarian Church of Transylvania, and the Oneness Pentecostals.