Interesting that you bring that point up. Those who hold to postmillennial ideology would naturally be inclined more towards this idea and many among the Reformed are so indeed of that persuasion. But one need not hold to that view, as did the great Jonathan Edwards, to arrive at a similar conclusion. Recently Dr. Danny Akin, an Evangelical Baptist Dispensationalist, who presides as president of the Southeastern Baptist Theological Seminary there in your state of NC, came to similar conclusions that Christians need to reproduce more than they are. However, of course neither he nor Edwards arrived at the same conclusions about polygyny. Well, I suppose Edwards has now that he is with the Lord, but Akin has yet to arrive to that conclusion. Practically they have the right idea, as you do, that with more children the more ground with the gospel can be covered, but the solid, astute, and brilliant Dr. Akin and of like company with him hit a problem because few and far between percentage wise can actually afford to have more than the standard one to two children due to financial constraints. So they are stuck in a difficult conundrum. They have the right idea but have not got the right structure to support the idea.
Yet on the other hand, some, more than I care to mention, among the polygyny tradition, grasp the right structure but so often they get so fast paced that when they do form the right structure they fail to have the right theological or spiritual maturity behind them to sustain what they have initiated. Or, some get the right theology but then embrace a form of separatist fundamentalism that leads them towards isolationism instead of evangelical engagement. Of these some fail to make the gospel the priority and make something lower on the doctrinal scale the key. Thus they are in a conundrum because they produce a lot of children who are not spiritual or believers in the gospel. So the goal of having more children to cover more ground with the gospel is undermined and it turns out to be nothing more than just more people without the passion for the gospel.
The solution is for there to arise a solid group of solid, well educated, evangelically passionate, mature, theologically astute set of saints that see the holistic picture: (1) The need to produce as many children in a responsible fashion according to their means and abilities without over extending their emotional and physical stewardship means (with polygyny being accepted as a norm) and (2) the need to not seclude themselves from the culture in a neo-monastic secluded bubble culture but instead are engaging the culture as Christ engaged the culture and as the early saints engaged the culture with the pure gospel of grace driving the whole enterprise which (3) has the strength of solid financial backers from those who have achieved greater wealth to help fund the mission through the application of godly biblical principles for the structure of the family as both a missiological center as well as a financial hub for missiological endeavors.
Hopefully, we will make headway towards the solution herein along with other endeavors that the Lord orchestrates through his sovereign plan as history progresses forward towards the ultimate culmination point.
Dr. Allen