The word heresy comes from haeresis, a Latin transliteration of the Greek word meaning choosing, choice, course of action, or in an extended sense school of thought. Irenaeus (b. between the years 115 – 125 according to some, or 130 – 142, d. 202) defined heresy as deviation from the standard of sound doctrine.
The Ante-Nicene Period (“before Nicaea”), or Post-Apostolic Period, of the history of early Christianity spanned the late first century to the early fourth century, with the end marked by the First Council of Nicaea in 325. Christianity during this time was extremely diverse, with many developments difficult to trace and follow. There is also a relative paucity of available material and this period is less studied than the preceding Apostolic Age and historical ages following it.