I've been teaching a section on Pelagianism lately and it's got me thinking again about the complimentary aspects of nature and grace. I was once a Calvinist in my Protestant days and I was fond of labeling people as Arminian (those who deny the Calvinist doctrine of unconditional election, etc.) However, the worst label one might receive is "Pelagian". If you were "Pelagian" then that meant you were living a life of "works righteousness" and trying to earn your way to heaven.
However, that's not truly the Pelagian doctrine. If you read Augustine and the canons of the Synod of Orange (AD 529), you'll see that the debate was primarily over the nature of grace and the purpose of sacramental baptism.
Calvinists, in their zeal to be "non-Pelagian" actually make several Pelagian moves. First, they confuse nature and grace. Second, they redefine the doctrine of original sin in accordance with their first mistake. Thirdly, they discount baptism as essentially doing nothing (other than marking a covenantal boundary). Fourth, each desire a pure Church of rigor in which moral corruption is not tolerated. All four are moves in the Pelagian direction. I realize that there are sophisticated "tractarian Calvinists" who could steer through these objections, but I think that Calvin's anthropology (especially Luther's) is essentially Pelagian.
Luther saw that raw human nature could never be transformed and so he sought a system of legal fiction by which God would impute righteousness to a believer that was not actually inherent in the believer. Pelagius also didn't believe that grace could transform a raw nature and so he understood "grace" to be the example and pattern of Christ. Luther's system is merely and exhausted Pelagianism. A throwing up of the hands.
What is missing in both is the Catholic doctrine stating that grace perfects nature. Nature cannot be redeemed apart from the perfection of grace transforming nature. Both Pelagianism and magisterial Protestantism believe that man is saved without the transformation of his nature. In his nature, he must simply try hard (Pelagius) or simply rely on a forensic declaration of God (Luther and Calvin).
The canons of the Council of Trent point to another way. Justification is the infusion of grace, righteousness, faith, hope, and charity into the soul of a believer so that he is conformed to Christ actually and in his person. He becomes righteous because righteousness is imparted, not merely imputed.