Pelagius (355-425) taught that we possess an inherent ability to overcome sin, perform good works, and merit our salvation; and he denied that divine grace is necessary for salvation. Pelagians believed that grace facilitates salvation, but that salvation does not require it.
St. Augustine (354-430) in Book II of his treatise De Peccatorum meritis et remissione (On the Merits and Remission of Sins) complains that an inevitable consequence of the assertion that we require no divine assistance is that it obviates the need to pray that we be not “overcome of temptation, either when it deceives and surprises us in our ignorance, or when it presses and importunes us in our weakness”; and it causes us to think that the petition “Lead us not into temptation,” is “a vain and useless insertion” to the Lord’s Prayer. (II, 2)
He elaborates: “For the commission of sin we get no help from God; but we are not able to do justly, and to fulfil the law of righteousness in every part thereof, except we are helped by God. For as the bodily eye is not helped by the light to turn away therefrom shut or averted, but is helped by it to see, and cannot see at all unless it help it; so God, who is the light of the inner man, helps our mental sight, in order that we may do some good, not according to our own, but according to His righteousness. But if we turn away from Him, it is our own act; we then are wise according to the flesh, we then consent to the concupiscence of the flesh for unlawful deeds. When we turn to Him, therefore, God helps us; when we turn away from Him, He forsakes us.”
Before God gives the grace that helps us to perform good works, He gives us prevenient grace, which helps us to ask for His assistance: “He helps us even to turn to Him; and this, certainly, is something that light does not do for the eyes of the body.”
St. Augustine gives these illustrations of grace in action: “When, therefore, He commands us in the words, ‘Turn ye unto me, and I will turn unto you’ (Zec 1:3), and we say to Him, ‘Turn us, O God of our salvation’ (Ps 85:4), and again, ‘Turn us, O God of hosts’ (Ps 80:3); what else do we say than, ‘Give what Thou commandest?’ . . . When He commands us, saying, ‘Do justice’ (Is 56:1), and we say, ‘Teach me Thy judgments, O Lord’ (Ps 119:108); what else do we say than, ‘Give what Thou commandest?'” (II, 5)
Finally, St. Augustine makes this plea: “Let us then drive away from our ears and minds those who say that we ought to accept the determination of our own free will and not pray God to help us not to sin.” (II, 6)
Quotations from A Select Library of the Nicene and Post-Nicene Fathers of the Christian Church, Vol. V, ed. Philip Schaff (Edinburgh: T&T Clark, 1886).