Augustine of Hippo rose to the challenge of opposing Pelagius during the early fifth century when Pelagius denied the necessity of grace for salvation. Although Pelagius believed Adam sinned in Eden, he did not see that first sin as having any radical consequences for Adam’s descendants. It did not affect our ability to obey the Lord. Grace might be helpful, but it was not a necessity, for God’s issuing of commandments even after the fall means that we are able in ourselves to keep them. Our free will—the equal ability to choose between good and evil—remained intact.
The great North African church leader could not be silent when he learned of Pelagius’ teaching, for he had a far better grasp of biblical teaching than Pelagius did. In responding to Pelagius, Augustine formulated one of the most extensive treatments of original sin—how Adam’s sin affects his natural descendants—in church history. Augustine faulted Pelagius for failing to make a distinction between free will and liberty. Free will, Augustine said, is simply the ability to make choices among several options, and human beings retain it after Adam’s fall into transgression. What we lack apart from grace, however, is liberty. According to Augustine, liberty is the ability to choose what is good and pleasing to the Lord. True freedom consists in doing what our Creator approves of. So, Augustine said, we may have free will after Adam, but we are not truly free apart from grace.
Following Scripture, Augustine described the fourfold state of humanity. Prior to the fall, we were able to sin or not to sin (posse peccare, posse non peccare). We could resist temptation and obey God’s command not to eat the forbidden fruit (Gen. 2:15–17). But in the fall, we lost that ability. Now, we are not able not to sin (non posse non peccare). In other words, without transformation by divine grace, the thoughts of our hearts are only evil continually (6:5). We are dead in sin and trespasses until the Lord sovereignly intervenes to give us new spiritual life (Eph. 2:1–10). At that point, we regain the ability not to sin (posse non peccare), but that does not mean we will be sinless. Until we are brought into God’s presence, sin remains, and we will succumb to temptation from time to time. We engage in the lifelong battle against sin that Paul describes in Romans 7.
In our glorification, however, we will enjoy the truest freedom possible, for we will be unable to sin (non posse peccare). We will practice only righteousness and will shine with the brightness of the stars, for we will be fully conformed to Christ (Dan. 12:3; Phil. 3:21).