It is undoubtedly no accident that Augustine's discussion of election and reprobation comes with his discussion of the historical moment of the Gospel. What we see when we look at the church in this world is not what it will be in glory, but rather a wide net which includes believers and unbelievers alike who will not be truly separated until heaven. The Incarnation, Crucifixion, and Resurrection are the culmination of history and the good news which is declared to all nations and in all tongues. Though the church is not yet pure as it will be one day, history's purpose is accomplished and we only await that final separation at the end of time.
How does the church grow? By the preaching of the Word, as confirmed by miracles and the death of the martyrs. This goes on until the very people who executed the martyrs come to respect them as forerunners in the faith and see their former pagan gods as now worthy of persecution.
Whether we're thinking about heresy or persecution, even the bad things of this world which plague the church are used by God for the good of His true people. "So it falls out that in this world, in evil days like these, the church walks onward like a wayfarer stricken by the world's hostility, but comforted by the mercy of God."