What I call "optimillennialism" ("OM" for short) here is very broadly defined. It includes any eschatological position that believes Christ to be ruling now; that the Kingdom has already come in some sense, is increasing through history and will be completed within the present historical epoch. There is also the corollary that Christians play a role in the increasing manifestation of the Kingdom by actively working for it. Such a position covers postmillennialism and most forms of preterism. The differences between these various positions can be ignored for the present purpose.
Basically,OM take two forms: theonomic or "top down" and (what might be termed) revivalist/pietistic/mystical or "bottom up". A prominent exponent of the former is Rushdoony and, of the latter, Jonathan Edwards.
The vision of the former is the transformation or reconstruction of society along Christian lines as brought about by Christians in government who, in effect, legislate for Christian morality as the basis of society. It aims at establishing a theocracy in which Christian rulers are said to be the viceroys of God. The Old Testament theocracy is, in essence, their model for the Millennium.
By contrast, the vision of the alternative form is for the building up of Christian influence within society through the conversion of individuals. As more people turn to Christ, so "Christian morality" will increase within society. This will be reflected both in who is elected to government (in a democracy) and in the sort of legislation introduced and supported.
In my opinion, the biggest problem with the first form is that it assumes that godliness can be legislated into existence. But the theocracy of the Old Testament failed. I believe that it was destined to fail, because only by demonstrating its weaknesses could human beings be taught that they are totally incapable of acting as God's viceroys. It is probably noteworthy that in the New Testament, Jesus never attempted to revive the old theocracy. In fact, he always refused to do so. Had he ridden a horse instead of a donkey into Jerusalem on what we now call Palm Sunday, history would have been very different. Horses were ridden by warriors and if Jesus had appeared as such, violent revolution would surely have broken out and he would have been proclaimed King. It is worth pondering that it was Satan who offered Jesus the position of ruling the kingdoms of the world!
Concerning the bottom up OM, it is an historical fact that great revivals can have profound effects on wider society. Note how the early Methodist awakening led to social reform in England and how the villages of Wales responded to the great revival that swept through the land during the first decade of last century. Nevertheless, it is also true that revivals tend to be like meteors, lighting up the landscape for a short while and then fading away. They can also become inward looking. Indeed, much revival preaching today seems more inclined to look away from the wider world and into ones own life in a manner that can become too one-sided.
My own position is basically bottom up, but there are some theological points which need to be addressed (and are seldom addressed) by all OM believers.
First, I do not believe that God ever intended human beings to be his viceroys (as at least some top down folk explicitly teach). A viceroy is an individual governor who rules as a representative of an absent king. The Viceroy of India, for example, ruled in place of the British monarch who was thousands of miles away and had little if any immediate knowledge of what was happening in India. But God is NOT and absent Monarch! He needs no "governor" to represent him as he rules directly. Furthermore, in the human realm, both monarchs and their viceroys are equal in each being independent persons. Only in their occupations are they very different. But God is unique. We may have been created in his image, but we are his creature totally depending upon him. He is the only complete Person. We derive our "personhood" from him. Picture the Sun and the Moon. Only the Sun produces light in the full sense. Strictly speaking, there is no such thing as moonlight - it is only sunlight reflected by the surface of the Moon. So are we only persons in so far as we reflect the Personhood of God.
We were created, not to be viceroys of God but vessels capable of containing the Diving Presence through whom God acts within human society. The church (i.e. the company of Christians, both locally and universally) is the Body or the Vine through which God in Christ acts in human society. The ideal is for each Christian and the church corporately to be so inwardly convicted of the individual as the vessel in which Christ is present that society increasingly becomes the Body of Christ ruled by the one Person; the one Self controlling the Body and each organ of the Body - the One who is the life of the Vine and each branch thereof.
That, I believe, is what God is calling us to realize, AND TO ACT UPON, in this age. Not just a society in which Christians are numerically dominant, but a society filled with Christians who are inwardly convicted that each "lives, yet not I but Christ lives in me". Christians having this conviction can come together and pray for collective guidance regarding any matter - social or otherwise. Only when each member of the group is inwardly convicted that a particular course of action is God's will, will this then be acted upon. Inward conviction - not "the will of the majority" or even mere consensus, but deep convicted certainty given by the Holy Spirit. Some day, I believe, the governments of the world will be run that way. Then, through his totally surrendered vessels, Christ will rule completely!