Why be a postmillenialialist?
My standard line on this is that all versions of historical premillenialism are less likely to be viable readings of Biblical eschatology than non-premillenial views and that among such views, the best versions of postmillenialism (postmil) are not significantly different from the best versions of amillenialism (amil) so that there is no real choice to be made between them. As a result, the best view of eschatology is the classical anti-chiliastic stance that the church has adopted since Augustine.
Given that, the torturous debates between amils and postmils in the last two centuries have been puzzling and interesting. I think the sources of explanation for them are primarily cultural (Scottish formulations versus Dutch formulations) and historical (the sudden shift of evangelicalism from being a dominant position in worldwide Victorian culture in the 19th century to its dramatic marginalization in the twentieth century along with that same culture in societies formerly possesing both). However, most of the questions properly try to difine and distinguish the two views and recommend one over the other on the basis of biblical exegesis. I want to suggest that, given that some antichiliast position seems to be likely to be most correct, it may not be necessary to go to such detailed exegetical work to decide between postmil and amil.
The reason is that there is an important similarity between the problem of deciding between postmil and amil and the problem of evil. Both positions agree that the figure of the Millenium points to the particular work of the Trinity in heaven and earth appropriate for the period between the two comings of Christ, but differ in that amils think that period is to be characterized by the church in trial and suffering and the opposing world in ascension, and postmils think the time will be characterized by the church prospering and the opposing world restrained. Neither side necessarily sees the decline or progress as exclusively happening and both allow intermittency between the two. Yet is seems that both sides agree that it is hard to see how an apparently prosperous enemy can be reconciled with a supposedly prosperous kingdom. And this parallels the problem of evil namely how can the existence of a good God be reconciled with the presence of evil. In particular, the amil/postmil tension seems to especially resemble the philosophical problem of the amount of evil, namely that even if the problem of evil per se was not a real problem, the fact that there is so much evil when there could have been less is irreconcilable with a the existence of a good God. In the case of the millenial question, it seems like the amils are the ones saying that the greatness of persecution and secularization is too much to accept a notion of the kingdom that is generally prosperous and the postmils are saying that since the kingdom is prosperous the amount of success of secularization and persecution must be less than it appears.
My response is that if we find the responses to the problem of the quantity of evil in apologetics to be adequate we should also think that the same sort of response is adequate in defence of an essentially prosperous kingdom of God. The basic reply in apologetics is to deny that we are in any position to assess how much evil would be illegitimate since there is a natural knowledge gap betweem us and God. Consequently nothing compels us to believe that their is too much evil. The same point could also be made in eschatology -- we are in no position to evaluate whether or not their is too much secularization et. al. to deny the existence of a prospering kingdom so there is no reason not to be a postmillenialist. This even applies given that we find both kinds of prophecies in the Bible -- both of prosperity and persecution. Future badness can be foretold and yet this does not necessarily count as evidence against a triumphant kingdom. The solution is clear: if one is a theist and one is an anti-chiliast, then one should be a postmillenialist.
Two further points: One, I think this answer serves an an excellent defense for being a Warfieldian in eschatology. Warfield's solution famously was to argue that Rev 20 specificly refers to events in the heavenly realm rather than on earth so that specificly the millenium has nothing to do with the church's experience in time, while he yet maintained the traditional Old Princeton optimism about the church based on other passages of Scripture.
Two, I think the disputes between postmils and amils may (but not must) indicate a certain hypocracy in that we offer a rationale to our non-Christian interlocutors who confront us with the problem of evil that we ourselves do not really find compelling in eschatology. We need to make up our minds.