Every kingdom has a king. Perhaps one of the main roles of a king which stands out, when considering a kingdom Biblically, is that a king is the one who delivers the people from their enemies. In the book of Samuel, the people of Israel made Saul king. Samuel had this to say about it:
[In Israel's history, God...] sold them into the hand of Sisera, commander of the army of Hazor, and into the hand of the Philistines, and into the hand of the king of Moab. And they fought against them. And they cried out to the LORD and said, “We have sinned, because we have forsaken the LORD and have served the Baals and the Ashtaroth [pagan gods]. But now deliver us out of the hand of our enemies, that we may serve you.” And the LORD sent Jerubbaal and Barak and Jephthah and Samuel and delivered you out of the hand of your enemies on every side, and you lived in safety. And when you saw that Nahash the king of the Ammonites came against you [most recently in the narrative], you said to me, “No, but a king shall reign over us”, when the LORD your God was your king. (1 Samuel 12:9-12)
We see here how God, when the people put their trust in Him, delivered them from their enemies. As king, God was their deliverer. God raised up people like Jerubbaal and Barak to deliver Israel; but here Israel is not turning to God to raise up a deliverer, instead they are putting their trust in Saul.
All the Earth belongs to God, and He is “King of Kings and Lord of Lords” (Revelation 19:16). God has always been king, and we are presently in God's kingdom. However, as you can plainly observe in the world around you, not many acknowledge God as the king. In the present time there is another “ruler of this world” who would usurp the throne of God. This is the Devil. When Christ came to the Earth, realize that the king walked amongst us. And the king was put to death by the “children of the Devil”...
“...but now you seek to kill me … You are of your father the devil, and your will is to do your father's desires. He was a murderer from the beginning...” (John 8:40,44)
The Bible tends to put all people into one of two categories; “children of God” or “children of the Devil”. In God's kingdom, God's people acknowledge Him as king and serve Him. But also in God's kingdom, God's enemies reside and ultimately serve the Devil in opposition to the true king. However, some of the enemies of God “switch sides”. To believers, Paul writes:
Therefore, since we have been justified by faith, we have peace with God through our Lord Jesus Christ. ... For if while we were enemies we were reconciled to God by the death of his Son, much more, now that we are reconciled, shall we be saved by his life. (Romans 5:1,10)
This “peace with God” is that kind of peace we see when warring parties are reconciled. This reconciliation is achieved through the cross, which is why the cross is referred to as Christ's victory over His enemies...
And you, who were dead in your trespasses and the uncircumcision of your flesh, God made alive together with him, having forgiven us all our trespasses, by canceling the record of debt that stood against us with its legal demands. This he set aside, nailing it to the cross. He disarmed the rulers and authorities and put them to open shame, by triumphing over them in him. (Colossians 2:13-15)
So Christ, the king, is about the kingly business of delivering His people from their enemies. The cross was the start of Christ's victory, but the completion is not yet. At present there are many enemies of God. When Christ returns to this Earth, then He will fully and finally destroy all of the enemies of the kingdom.
But each in his own order: Christ the firstfruits, then at his coming those who belong to Christ. Then comes the end, when he delivers the kingdom to God the Father after destroying every rule and every authority and power. For he must reign until he has put all his enemies under his feet. The last enemy to be destroyed is death. (1 Corinthians 15:23-26)
So we see the kingdom at this present time as a kingdom in which there are many loyal servants of the king, and many others who hate the king and refuse to serve Him, and instead they serve another master. But the true king will one day remove such people from His kingdom, for He is the rightful king regardless of how many people don't like Him. We can consider the parable in Luke 19 which begins “A nobleman went into a far country to receive for himself a kingdom and then return.” We also read “But his citizens hated him and sent a delegation after him, saying, 'We do not want this man to reign over us.'” Now, the king returns and deals with his servants accordingly, but the parable finishes “as for these enemies of mine, who did not want me to reign over them, bring them here and slaughter them before me.” (Luke 19:12-27) This parable describes the return of Christ, the king.
For as the lightning flashes and lights up the sky from one side to the other, so will the Son of Man be in his day. But first he must suffer many things and be rejected by this generation. (Luke 17:24-25)