Rather than appearing in its pure and simple form, Pelagianism has been far more prevalent in church history when it has been mixed in with a bit of grace.
Cornelis Venema, of Mid-America Reformed Seminary, in his recent book dealing with the New Perspective, argues that E. P. Sanders portrayal of Second-Temple Judaism has strong affinities with semi-Pelagianism:
The glaring weakness of Sanders' case...is that he does not consider whether 'covenantal nomism' could accommodate a form of religious teaching that regards salvation and acceptance with God as being based on grace plus good works.
Covenantal nomism is a sufficiently elastic pattern for the religion of Second-Temple Judaism that it could express a kind of a semi-Pelagian view of the relation between God and his people.
That Second-Temple Judaism was not full-blown Pelagianism is not surprising. In the course of history, Pelagianism is a 'rare bird' in the aviary of Jewish and Christian theology. Few have argued that salvation does not require the initiative of God's grace but is simply based on human moral achievement.
Where Pelagianism has appeared, therefore, it has commonly been condemned by the major branches of the Christian church. Semi-Pelagian views, however, are quite often found in the history of Christian theology. Though these views may speak of God's gracious initiative in salvation, they also insist that human salvation does not end with a good beginning.
According to semi-Pelagianism, those who find favour and acceptance with God are those who freely co-operate with his grace and complement it by a life of good works that merits further grace and final salvation.
Cornelis Venema, The Gospel of Free Acceptance in Christ: An assessment of the Reformation and the New Perspective on Paul, p. 156-7