Isn't that Pelagianism?
When I explain to people about what the early church believed in the second to fourth centuries AD, they sometimes make the comment "Isn't that Pelagianism?" I'm never quite sure how to respond to that one. I've come to think the simplest and least misleading answer is "basically yes".
Unfortunately this can inspire people to ask truly nasty follow-up questions like "so why was Pelagianism condemned?"... which doesn't admit of anything approaching a simple answer other than perhaps "you're assuming things happen sensibly for rational reasons". Other than that you have to start tracing the convoluted doctrinal development in the 4th-5th century West that lead to Augustine's radical innovations, and then explain the historical and political circumstances that lead to Augustine's influence in the West such that he managed to get a defender of orthodoxy condemned in order to attempt to answer this question. So perhaps the short answer to that is that Pelagius was condemned because he dared correctly label the most influential person in Western Christendom at the time as an innovating heretic. (This is not of course to claim that Pelagius was totally free from all errors whatsoever.)
I had a read of a couple of the standard reference works over the weekend to see if they had anything positive to add that would help me explain things to people who ask me about Pelagianism. They weren't exactly helpful, both taking the view that Pelagianism was substantially identical to the orthodoxy of this period. The Lutheran writer Jaroslav Pelikan in his The Emergence of the Catholic Tradition (100-600) comments:
In the orthodoxy of the second to fourth century we encounter "Pelagianism before Pelagius." (285)
"Much of this [Celestius' Pelagian doctrines] could claim support from the tradition as well as from contemporary Eastern theologians." (316)
"An injustice may have been done here [in the condemnation of Pelagianism] as in other dogmatic debates, but it was an injustice that made history." (313)
The Anglican writer JND Kelly in his Early Christian Doctrines explains the development of doctrine over this period, noting a virtually non-existence difference between orthodoxy and Pelagius' views, and analyzing Augustine's deviations. He ends up wryly commenting that this all might warrant the view that "the Eastern attitude generally is to be dismissed as Pelagianism." (373)
Of course, all that doesn't really help me answer people's questions on the subject.