I think if you read the quote in context carefully, Augustine is not talking about "losing grace" as in finding oneself outside the New Covenant, after once being in Christ. Even if you look at the title to the chapter, it contains, "the grace of obedience." You may reject that particular grace for a season at your own fault, and yet not such that you are severed from Christ. The context is about restoring an erring believer by rebuke. He even intimates that if the believer rebuked is a "child of promise" it ought be effective.
Hence, he says later (again in ch. 23), "they who for a season wander from the way return, that they may continue unto the end what they had begun to be in good."
