The parable's point: repentant tax collectors and prostitutes are justified
Theophilus is explaining the point of the parable of the two sons from Matthew 21:28–32, now stating what the parable means for the first group.
This is what the parable wants to say: the tax collectors and the prostitutes repented and did the things pleasing to God, and so they are justified, having been persuaded by John—even though at first they were disobeying the law, or rather, disobeying God himself.