If the wages of sin is death, why do forgiven people still die? Augustine does not say, “Because even forgiven people are still guilty.” Rather, Augustine suggests death looms for the forgiven man so that he may gain in righteousness.
If any person who became baptized need no longer fear death, “faith itself would be weakened, since faith is only faith when what is not yet seen in reality is awaited in hope… faith would not be tested by the fact its reward is unseen; indeed, it would not be faith any longer, since the reward of the act of faith would be demanded…” (City, Book XIII.4)
Faith is a virtue. When Augustine writes that certainty of eternal life weakens faith, he implies that incentive for virtue dries up when the reward is enjoyed prior to the completion of the labor. Put simply, a human being is simply too weak to bear the burden of certainty.
However, that quality of life on earth which is most vexing to Augustine is uncertainty. Will I apostatize? Will my children leave the faith? Will I die suddenly by foreign invasion, plague, or sword? Man cannot conceive of the future. He cannot say with certainty that he has overcome any sin, including the sin of rejecting God. Thus, life on earth is fraught with deep anxiety. However, Augustine seems to reason as such: God could either command man to live with eternal certainty or eternal uncertainty. Because God loves man, He grants man a kind of knowledge best suited to his weakness. While uncertainty is a burden, it is a lighter burden than that of certainty, for the suffering of uncertainty tends toward virtue, while the suffering of certainty tends toward sloth.