Augustine of Hippo, On the Grace of Christ, and on Original Sin I.31: Pelagius and Cœlestius Nowhere Really Acknowledge Grace

Let him amend all this, that if human infirmity has erred in subjects so profound, he may not add to the error diabolical deception and wilfulness, either by denying what he has really believed, or by maintaining what he has rashly believed, after he has once discovered, on recollecting the light of truth, that he ought never to have so believed. As for that grace, indeed, by which we are justified,—in other words, whereby “the love of God is shed abroad in our hearts by the Holy Ghost, which is given unto us,”—I have nowhere, in those writings of Pelagius and Cœlestius which I have had the opportunity of reading, found them acknowledging it as it ought to be acknowledged. In no passage at all have I observed them recognising “the children of the promise,” concerning whom the apostle thus speaks: “They which are children of the flesh, these are not the children of God; but the children of the promise are counted for the seed.” For that which God promises we do not ourselves bring about by our own choice or natural power, but He Himself effects it by grace.