Christ reigning as King over all things does not mean the immediate eradication of sin, self, and Satan.
1 Corinthians 15:24-28 24 Then comes the end, when He delivers the kingdom to God the Father, when He puts an end to all rule and all authority and power. 25 For He must reign till He has put all enemies under His feet. 26 The last enemy that will be destroyed is death. 27 For "He has put all things under His feet." But when He says "all things are put under Him," it is evident that He who put all things under Him is excepted. 28 Now when all things are made subject to Him, then the Son Himself will also be subject to Him who put all things under Him, that God may be all in all. (NKJV)
Hebrews 2:8-9 8 You have put all things in subjection under his feet." For in that He put all in subjection under him, He left nothing that is not put under him. But now we do not yet see all things put under him. 9 But we see Jesus, who was made a little lower than the angels, for the suffering of death crowned with glory and honor, that He, by the grace of God, might taste death for everyone. (NKJV)
1. Biblical Facts
Christ is Lord of all
Acts 2:36 36 "Therefore let all the house of Israel know assuredly that God has made this Jesus, whom you crucified, both Lord and Christ." (NKJV)
Ephesians 1:20-23 20 which He worked in Christ when He raised Him from the dead and seated Him at His right hand in the heavenly places, 21 far above all principality and power and might and dominion, and every name that is named, not only in this age but also in that which is to come. 22 And He put all things under His feet, and gave Him to be head over all things to the church, 23 which is His body, the fullness of Him who fills all in all. (NKJV)
Satan is Bound
Revelation 20:1-3 1 Then I saw an angel coming down from heaven, having the key to the bottomless pit and a great chain in his hand. 2 He laid hold of the dragon, that serpent of old, who is the Devil and Satan, and bound him for a thousand years; 3 and he cast him into the bottomless pit, and shut him up, and set a seal on him, so that he should deceive the nations no more till the thousand years were finished. But after these things he must be released for a little while. (NKJV)
Matthew 12:28-29 28 "But if I cast out demons by the Spirit of God, surely the kingdom of God has come upon you. 29 "Or how can one enter a strong man's house and plunder his goods, unless he first binds the strong man? And then he will plunder his house. (NKJV)
Christians are sanctified
Romans 6:11-12 11 Likewise you also, reckon yourselves to be dead indeed to sin, but alive to God in Christ Jesus our Lord. 12 Therefore do not let sin reign in your mortal body, that you should obey it in its lusts. (NKJV)
2. Empirical Reality
Sin still abounds
Satan is still active
Death and sickness are still experienced
3. Conclusions
The Kingdom is progressive - all will be subject!
Sin, Satan, death, and sickness will one day be completely destroyed
We will continue to suffer till that time
Hebrews 2:10 10 For it was fitting for Him, for whom are all things and by whom are all things, in bringing many sons to glory, to make the captain of their salvation perfect through sufferings. (NKJV)
"As Christ carries on war continually with various enemies, it is doubtless evident that he has no quiet possession of his kingdom. He is not, however, under the necessity of waging war; but it happens through his will that his enemies are not to be subdued till the last day, in order that we may be tried and proved by fresh exercises."
John Calvin