In one sense, the kingdom of Christ has already come. The birth of Jesus was the birth of the "king of the Jews" (Matt. 2:2). When the rightful king arrived in the midst of his people, the kingdom of God had already come. This was the entrance of the king.
Then, when Jesus began his public ministry, he declared that "The time is fulfilled, and the kingdom of God is at hand; repent and believe in the gospel" (Mark 1:15), and that "the kingdom of God is in the midst of you" (Luke 17:21). During his public ministry, the true King was already working for the establishment of his kingdom—a work culminating in his crucifixion, death, burial, and resurrection.
Now, Christ has ascended to his Father's right hand, where he is already reigning (Acts 2:33; Phil. 2:9–11). Until he returns again, King Jesus will continue the work he has already begun of the expansion of his kingdom throughout the whole world by the preaching of the gospel and the power of the Holy Spirit.
In another sense, though, the kingdom of Christ has not yet come. Not every rule, authority, and power has been yet put under his feet—especially death (1 Cor. 15:24–28). While Christ has done everything necessary to enter into the midst of his kingdom, to establish his kingdom, and to expand the rule of his reign throughout the world, the work is not yet complete.
His kingdom is both already and not yet.
Christ's Already/Not Yet Kingdom
We keenly feel the tension of living in the already/not yet kingdom of Christ. Sometimes, we despair to think that the already of Christ's kingdom has not yet come. We forget all that God has done in and through his Son Jesus Christ for the salvation of sinners by grace, through faith.
Other times, we are deceived to believe that the not yet has already come. We forget that there is much work to be done, and, in the course of that work, much sorrow and suffering to endure.
Paul captures the implications of the already/not yet nature of Christ's kingdom well in 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! (1 Cor. 4:8)
Clearly, Paul is using irony and sarcasm to rebuke the Corinthians for the fact that they are already satisfied, rich, and reigning. With this in mind, pay careful attention to Paul's point in this verse.
Paul is not complaining that the Corinthians have an easier life than he is experiencing. Rather, he is making a specific point about the nature of the kingdom of Christ.
Reigning with Christ Already
So, while many English translations render something like "become kings," the word Paul uses is the exact same word that he uses in the next sentence, which the ESV translates as "did reign." The idea has to do with entering into, or taking up, a reign. The latter translation gives the impression that we will reign as kings, while Paul is instead saying that we will reign with a King.
Paul is not, then, merely criticizing them simply for living like kings. Rather, he is making a very specific statement about their relationship to Christ's kingdom. That is, he is saying that they are living as though they have already taken up their reign with Christ. Elsewhere, Paul insists that we will reign with Christ: "If we endure, we will also reign with him" (2 Tim. 2:12).
The problem is not that the Corinthians believe that they will reign with Christ. Paul himself longs for that day: "And would that you did reign, so that we might share the rule with [lit: "that we might reign with"] you!"
Instead, the problem is that they are living as though they have already entered into Christ's not yet kingdom.
Waiting for the Not Yet
The already/not yet nature of Christ's kingdom means that we do not yet experience what Christ has already established for us in his kingdom:
Christians may expect to be satisfied, but not yet
Christians may expect infinite riches, but not yet
Christians may expect to reign with Christ, but not yet
It is not wrong to desire these blessings, for God indeed promises them. If we downplay, minimize, or deny that we will enter into joy and riches when the Father gives us the kingdom (cf. Luke 12:32), we misrepresent God's goodness.
On the other hand, if we insist upon these blessings already, we have fallen into the false "prosperity gospel." The reason the prosperity gospel is such a twisted perversion of the gospel is that it takes the infinitely good, not yet promises of God's kingdom, and makes them blessings we can expect already among the trivial, meager riches of this world.
The consequences of the prosperity gospel are devastating: by already embracing the corruptible riches and satisfactions of this world, we forfeit the incorruptible joy, satisfaction, and reign of Christ's not yet kingdom.
Living Now in Christ's Already/Not Yet Kingdom
How, then, should we live in the tension of Christ's already/not yet kingdom? Intriguingly, Paul uses the word "now" twice in the rest of this paragraph (vv. 11, 13), although our translations don't bring this repeated word out well:
 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.  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.  To the present [lit., "now"] hour we hunger and thirst, we are poorly dressed and buffeted and homeless,  and we labor, working with our own hands. When reviled, we bless; when persecuted, we endure;  when slandered, we entreat. We have become, and are still [lit., "now"], like the scum of the world, the refuse of all things. (1 Cor. 4:9–13)
The Cross of Christ
Living now in Christ's already/not yet kingdom looks less like satisfaction, riches, and reign, and more like the means by which Christ took up his own kingdom: by the cross. We receive joy through suffering, riches through poverty, reign through humiliation, and life through death.
Jesus himself offers us the perfect example of how to live now from his own earthly ministry. As he went, he declared that the kingdom of God was a hand; however, he lived in poverty, suffering, and rejection. Jesus endured the cross and despised its shame for the joy set before him—that is, for entering into his reign at the right hand of the throne of God (Heb. 12:1–2).
Taking Up Our Cross
Then, Christ tells us that we too must take up our cross to follow him (Matt. 10:38). Some of us will share in more of Christ's sufferings than others, but all of us are called to suffer now. Our suffering, however, is not in vain! By sharing in Christ's sufferings, we will ultimately share in his resurrection glory (Phil. 3:10–11; 20–21).
This reality should prepare us for the suffering we will endure and dismiss any foolish ideas about becoming prosperous already. More than that, however, this reality gives meaning to our suffering. When we suffer for Christ's sake, it is not because God has forgotten or abandoned us. Rather, our suffering now testifies to the reality of Christ's already/not yet kingdom.
Thus, we can rejoice even in the midst of the greatest suffering. Our King is already reigning, and our sufferings now are not even worth comparing with the glory that we have not yet received from him (2 Cor. 4:17–18).