‘In Piper’s own words he describes Augustine’s conversion. While listening to Ambrose, Catholic bishop in Milan, Augustine heard the voice of God, has no reason to doubt and loved God. Piper astonishingly says, But this experience was not true conversion. What was keeping Augustine from true conversion as affirmed by Piper? He knew that he was held back now not by anything intellectual, but by sexual lust.
So a person loves God, believes his Word and does not doubt but that is not sufficient for conversion. Is this not the definition of belief in God?
What is sufficient for conversion? Augustine clearly states and Piper clearly agrees with him that salvation by faith alone was not enough for conversion. Augustine had to purify himself from all desire for lust and sex in order to achieve salvation. Piper recognizes this salvation by works. He says Augustine after believing in God, loving God, having no doubt about was not saved.
As a reminder true conversion or salvation is by faith alone not works, and after believing Christ washes away all our, the sinner does not have to become sinless to become a Christian.
8 For by grace you have been saved through faith, and that not of yourselves; it is the gift of God, 9 not of works, lest anyone should boast.
13 In Him you also trusted, after you heard the word of truth, the gospel of your salvation; in whom also, having believed, you were sealed with the Holy Spirit of promise,
11 But that no one is justified by the law in the sight of God is evident, for “the just shall live by faith.”
If Augustine went through a true conversion he was saved when he believed. In his autobiography this would be Book VII, Chapter XVII and not his proclaimed conversion in Chapter VIII. What is missing from his conversion tale in Chapter VIII, is the witness of faith in Christ, which is strangely missing. The recognition that the death, burial, and resurrection of Jesus Christ cleanse the believer from sin.
Here is what Piper believes about Augustine’s conversion experience. Augustine already believed in God and loved God but was held back from conversion because he had not achieved a purification from the bestial bondage of lust. The battle was between not having sex and having sex. The final conversion was in the words of Augustine, You converted me to yourself, so that I no longer desired a wife.‘
source: Craig Fisher, “John Piper, the Profane” (craigcfisher).