The title well summarizes the contents of the book–it is a brief defense of optimistic eschatology and postmillennialism in particular. The book succeeds in being accessible, arguing for covenentalism, and showcasing the promises (optimism) of Scripture, but not so much in clearly demonstrating that postmillenialism is superior to alternative views.
Sandlin's basic support for postmillenialism stems from promises related to world-wide righteousness/peace/prosperity/etc. which will accompany the gradual growth of the kingdom as it marches forward according to the kingly rule of Christ and the mighty power of God at work in His people. The basic problem with the above support is, apart from a few minor details which Sandlin gives little attention to, amillenialists and even many historic premillenials would agree. The more important questions are: When will righteousness cover the earth? Before or after Christ comes? In what way will the kingdom grow? Through the growth of the church or necessarily politically/nationally as well? It is not enough to declare that the church will triumph--the when and how of that triumph are the important differentiators. These questions could have been hashed out in the chapters addressing premillennialism and amillenialism.
Sandlin argues that premillennialism arises primarily out of a mistaken view of Revelation 20:5-6, which from my understanding is itself mistaken. Premillenialism arises from the conviction that the Bible promises a glorious, yet future manifestation of the kingdom of God on the earth (Sandlin himself argues this point) and from the conviction that this manifestation of the kingdom follows the second return of Christ. These two beliefs arise from a much broader sampling of Scripture than a single difficult passage in Revelation, and again, only the later belief actually conflicts with postmillenialism. However, this understanding leads Sandlin to not take premillennialism very seriously. He devotes four pages to it and thinks that if he can poke a couple holes in their understanding of Revelation 20 he has undermined their entire eschatology.
Sandlin gives more care in addressing amillenialism, but his critiques are a little misplaced. The biggest disconnect I noticed is that for Sandlin, "the eternal state" or "eternity" is absolutely incompatible with "earthly" or "physical" promises. Therefore, to refute amillenialism he merely has to establish that the Bible has unrealized promises of peace, prosperity, and fruitfulness ("earthly" stuff related to animals, children, plants, houses, lands, etc.). If there are such promises Sandlin concludes, then they must be fulfilled on the physical earth prior to the establishment of the “eternal state”. However, because what he calls “the eternal state” is none other than a restored creation, the New Heavens and Earth, his argument is invalid. The argument that Sandlin needs to make is that there are biblical promises that cannot fit a perfected future earth (again, this is also one of the primary arguments for premillenialism), but he only follows such a line of reasoning for a single paragraph. He writes, “[These promises] cannot refer to the eternal state, since the symbolic discussion of life expectancy, agrarian labor, and nature’s increased harmony with man are not appropriate to the eternal state.” (pg. 28) Even in the above quotation a false dichotomy between the present creation and a future creation can be seen (i.e. agrarian labor being incompatible with our final dwelling place), but it is the closest he gets to making the right argument.
Interestingly, he also makes the case that the foundational protestant Confessions are optimistic in their eschatology. He argues this because, for example, the Westminster Larger Catechism states that Christ is “restraining and overcoming all [the church’s] enemies” (pg. 51). The Westminster Confession makes a very similar statement (8:8). I could be convinced otherwise, but I tend to see this as meager evidence to make such a claim. The authors of the confession were incredibly careful with their language–with what was included as well as what was excluded–and if they wanted to articulate the triumph of the church on the earth they would have done so clearly. The articulators of the 1689 London Baptist Confession of Faith actually added some clarifying statements that tend away from this as well. For example, they add this about Christ’s kingly rule: “Because we are hostile and utterly unable to return to God, and so that we can be rescued and made secure from our spiritual enemies, we need His kingly office to convince, subdue, draw, sustain, deliver and preserve us for His heavenly kingdom.” (8:10). The focus of Christ’s present kingly rule seems to focus on his authority and power at work in his people to cause their hearts to submit to him, and then preserve and strengthen their faith in the face of opposition. They also add a chapter on “The Gospel and the Extent of its Grace” and therein state “in every age the preaching of the gospel to individuals and nations has been granted in widely varying degrees of expansion and contraction, according to the counsel of the will of God.” (20:3) On the other hand, I think section V on the church in the Savoy does have a Post-mill flavor–it might be the case that it is inherently optimistic. On the whole, there isn’t enough evidence to substantiate Sandlin’s claim that the major historical protestant confessions favor postmillenialism/optimistic amillenialism.
I suppose I spent a lot of time critiquing A Postmillenial Primer, but I actually really enjoyed it and appreciated how succinct it is. I just think the author could've better identified how to best differentiate postmill from other views. I look forward to reading better and more detailed support for postmillennialism in the future.