Sometimes the term "Christendom" is unhelpful because many people refer to it in varied ways. Predominantly, a "Christendom" has been understood in terms of a type of rule of the church within a national identity in the world. The Roman church from the time of Constantine was a building "Christendom." Often Christendom would involve a blend of church and state rather than a separation of church and state. Today, some people use the term "Christendom" simply to suggest the countries where the prominent religion is Christianity. Either way, the notion of a Christendom has extended beyond the bounds of those who are regenerate believers to a cultural rule or predominate influence of "Christian" values. The problem is, often Christendom blends Christianity with culture and breeds compromise. A Christians' desire for prominence in the world often leads to worldliness.
When it comes to the idea of dominion, rule, or kingship, it seems to me that the Apostle Paul never had a "Christendom" in mind. Paul was very careful to identify the church as those who rule in the way they represent the saving power of Christ in eternal salvation. Any kingship in the church was not to be blended with the world but to be seen in contrast to it.
Take for instance Paul's first letter to the Corinthians. The Corinthian church was obviously full of problems, but those problems seemed to stem from worldliness and pride. They had to be reminded in the very first chapter that the pride and arrogance of worldliness is not the way of Christ. 1 Corinthians 1:27-29 But God chose what is foolish in the world to shame the wise; God chose what is weak in the world to shame the strong; 28 God chose what is low and despised in the world, even things that are not, to bring to nothing things that are, 29 so that no human being might boast in the presence of God. The power of God over this world is seen through what this world despises. The cross is God's dominion over the power of sin and death. Through suffering, Christ brings the restoration of creation and the salvation of his church.
Later in his letter, Paul revisits the pride of Corinth who so easily sit in judgment of his own ministry. 1 Corinthians 4:1-4 This is how one should regard us, as servants of Christ and stewards of the mysteries of God. 2 Moreover, it is required of stewards that they be found faithful. 3 But with me it is a very small thing that I should be judged by you or by any human court. In fact, I do not even judge myself. 4 For I am not aware of anything against myself, but I am not thereby acquitted. It is the Lord who judges me. Is Paul being effective? Is he ministering in a way that promotes Christianity in the world with real potency? These are the same questions asked about ministries today. So often criticism is leveled at ministers who will not take arguments to the streets (or social media) for the sake of standing up for Christian values. Why is the church not more politically motivated? Why is the church not more culturally charged? Why is the church not willing to fight in the public sphere on explicitly moral issues? Why is the church not in a better place of prominence? Paul's answer for the Corinthians is that his stewardship of ministry will be judged by God. His ministry is clearly the folly of the cross.
After rebuking the Corinthians for their worldly arrogance (vs. 6-7), Paul describes where their arrogance has taken them in a worldly sense. 1 Corinthians 4:8 Already you have all you want! Already you have become rich! Without us you have become kings! And would that you did reign, so that we might share the rule with you! Paul is actually saying that the Corinthians have become worldly in seeking power, prestige or even prominence in the world. Their idea of being kings is that Christians should have prominent place in the world. Paul says, if they really were kings in the world, he might join them. The reality is that this is not the Christian calling.
Instead, Paul shows them the ministry and calling God has for his servants in the ministry of the gospel. It's far from the Corinthian idea of kingship. 1 Corinthians 4:9-14 For I think that God has exhibited us apostles as last of all, like men sentenced to death, because we have become a spectacle to the world, to angels, and to men. 10 We are fools for Christ's sake, but you are wise in Christ. We are weak, but you are strong. You are held in honor, but we in disrepute. 11 To the present hour we hunger and thirst, we are poorly dressed and buffeted and homeless, 12 and we labor, working with our own hands. When reviled, we bless; when persecuted, we endure; 13 when slandered, we entreat. We have become, and are still, like the scum of the world, the refuse of all things. 14 I do not write these things to make you ashamed, but to admonish you as my beloved children.
If the church has a place of prominence in the world, it's more likely something to be concerned about. Paul is quite sarcastic about their strength and honor and wisdom in the world's measures. It is the gospel that offends. We are not called for cultural dominance; we are called into what is seen as foolishness to the world. We are called to minister the gospel and be willing to suffer because of it. It is the cross that will always be despised by those who are perishing. Rather than seeking to be kings, the Corinthians should seek a ministry that might end up in the very opposite circumstance. The response to the world's criticisms is not to fight for better position or ascendancy, but to bless, endure, entreat. All of this for the sake of Christ and his gospel.
In contrast to a Christendom, we are called to a humble ministry of the gospel. Yes, it may put us at odds with our culture, but not to fight for moral reform and political rule but because we simply live and preach Christ crucified.
Paul doesn't want you to be worldly kings, he wants you to be faithful servants of the gospel for the King of kings.