My own understanding of the church/state question begins with Romans 1. Paul describes the condition of the world outside the church to demonstrate that we need a Savior.
21 For although they knew God, they neither glorified him as God nor gave thanks to him, but their thinking became futile and their foolish hearts were darkened. 22 Although they claimed to be wise, they became fools 23 and exchanged the glory of the immortal God for images made to look like mortal man and birds and animals and reptiles.
24 Therefore God gave them over in the sinful desires of their hearts to sexual impurity for the degrading of their bodies with one another. 25 They exchanged the truth of God for a lie, and worshiped and served created things rather than the Creator–who is forever praised. Amen.
26 Because of this, God gave them over to shameful lusts. Even their women exchanged natural relations for unnatural ones. 27 In the same way the men also abandoned natural relations with women and were inflamed with lust for one another. Men committed indecent acts with other men, and received in themselves the due penalty for their perversion.
28 Furthermore, since they did not think it worthwhile to retain the knowledge of God, he gave them over to a depraved mind, to do what ought not to be done. 29 They have become filled with every kind of wickedness, evil, greed and depravity. They are full of envy, murder, strife, deceit and malice. They are gossips, 30 slanderers, God-haters, insolent, arrogant and boastful; they invent ways of doing evil; they disobey their parents; 31 they are senseless, faithless, heartless, ruthless. 32 Although they know God’s righteous decree that those who do such things deserve death, they not only continue to do these very things but also approve of those who practice them.
Paul says that the world without God is sexually perverse and filled with many obvious sins because “God gave them over” to such wickedness. Just in case we weren’t paying attention, he says it three times.
Why did God do this? Why would God want the lost to act so horribly? Obviously, God’s desire is that the lost be saved, but so long as they are lost, it serves God’s purposes for the futility and self-destructiveness of their existence to be manifest. It serves God’s purpose for the lost to be obviously different from the saved.
Why did God send Jesus at a time whenthe Roman Empire was much like 21st Century America–filled with sexual license and perversion, even celebrating wickedness? I think Jesus came when he did because that was a time when men’s hearts were most open to the gospel. It was a time when the contrast between righteousness and worldliness was obvious, and a time when the need for the gospel was plain.
Therefore, if the church uses the power of government to make the lost act like the saved, wouldn’t we be working at cross purposes with God? Nothing in the Bible calls on the church to impose its will on the lost. Rather, we are to persuade the lost to saved (2 Cor. 5:11).