Eternal security, also known as "once saved, always saved" is the belief providing Christian believers with absolute assurance of their final salvation. Its development, particularly within Protestantism, has given rise to diverse interpretations, especially in relation with the defining aspects of theological determinism, libertarian free will and the significance of personal perseverance.
Before the Reformation, belief in forms of eternal security were anecdotal. Besides, in the early 5th century, the Augustinian view of predestination by predetermination emerged, though it did not endorse eternal security. By the 16th century, this concept became integrated into the theology of John Calvin and other reformers. Calvinist circles initially embraced eternal security as one of the practical interpretations of the doctrine of "perseverance of the saints". Over time, the term became a synonym of the Calvinist doctrine of perseverance independently of its practical interpretations.
In the early 20th century, eternal security started to become a defining doctrine of the Southern Baptist traditionalism. Around the same period, it also became part of Plymouth Brethren theology. Those two forms represents its predominant forms today. In the 1980s, the Free Grace movement voiced this doctrine independently of the notion of personal perseverance, with subsequent variations emerging such as the "Hyper-Grace" teaching.