"Now I have told you before it happens, so that when it happens, you may believe. 30 I will not speak much more with you, for the ruler of the world is coming, and he has nothing in Me" John 14:29-30
When Christ said this He was speaking to His disciples before the day of Passover knowing that the following day He would be betrayed by Judas and handed over to the Jewish officials. In the above scripture He is making reference to the Devil, which is Satan, pointing to the fact that prior to Christ's death and resurrection Satan was 'the ruler of the world'. The statement, 'he has nothing in me' is a plain indication from Jesus, stating that from His birth into human form and for the rest of all eternity not even an iota of His being was taken captive by the ways of the devil, that which leads to sin.
Jesus later says in John 16:7 "But I tell you the truth, it is to your advantage that I go away; for if I do not go away, the Helper (The Holy Spirit, see John 15:26) will not come to you; but if I go, I will send Him to you." Jesus was informing the disciples that unless He went into the underworld and rose again the Holy Spirit would not be able to come into the hearts of men to lead them in the way of righteousness as outlined in the person of Christ.
Why did Jesus have to die for the Holy Spirit to be given to those who ask?
The Holy Spirit is a part of God and indwells the Lord Jesus Christ, by this fact alone we know He is holy, pure and free from sin. Prior to Christ's death and Resurrection there was no legal way for a sinless, holy and divine part of God to be given to human beings, who by their very nature sin all the time. By Jesus descending into the underworld (in His spiritual body) as a sinless man, Satan and all his host had no legal right to keep Christ there and as a result He was able to ascend on the third day, not before he stripped the enemy of all their legal right to rule and reign in the unseen realm, not only that but upon exiting Sheol He took 'the keys of death and of Hades' Rev 1:18.
This is profound revelation on why for so long people have assumed Satan is still the ruler of this world, for so long we have not been told or paid any special attention to the work Christ did during His 3 days in the underworld. He wasn't sitting around watching the clock until 72hours had passed, He was finishing the work God the Father had given Him to carry out on behalf of all mankind. The reason as to why Hollywood and so many other high profile institutions of power have disseminated ungodly, sin-filled content for so long is because at the highest level these people have sold their soul to the devil. In the same way that we believe Jesus is the way, the truth and the the life; the highest rulers in society believe in hedonism, sensuality and power and status in this life.
There is a very specific reason Christ said in John 17:14-15 "I have given them Your word; and the world has hated them, because they are not of the world, even as I am not of the world. 15 I do not ask You to take them out of the world, but to keep them out of the power of the evil one." Jesus said this during the 'High Priestly Prayer' in which He was speaking directly to the Father, the reason He said this was because after spending 33 years in a sinful body in fellowship with human's he makes it clear that there are many of us who desire to be close to the Father, and that not all of us submit to the temptations of the world. It's also important to highlight him saying, 'I do not ask You to take them out of the world, but to keep them out of the power of the evil one'. He's not asking the Father to remove us from the world and separate us from society, only that we be given divine protection in the heavenly realm from the schemes of the devil in whatever form they take.
We have to remember that the devil's legal right to have power over mankind ended the moment Christ rose on the third day with 'the keys of death and of Hades' in His hand Rev 1:18. This is what Christ's sacrifice truly represents. Because He was for 33 years in a human body exposed to the same temptations we all face and yet did not sin, His entry into Hell was for one purpose and one purpose only, to strip Satan of any legal right to influence and disrupt the lives of human beings. As a result of this removal of power for Satan we through faith and faith alone, are now able to receive the Holy Spirit of God which is ultimately the seal which God has set upon those who will be going to heaven. 2 Co 1:21-22 states, 'Now He who establishes us with you in Christ and anointed us is God, 22 who also sealed us and gave us the Spirit in our hearts as a pledge/down payment'. This is the seal of the Holy Spirit given to those who delight in following Christ's ways and live according to the two most important commandments, see - Matt 22:36-40.
Now it's a different matter entirely as to why Satan still has dominion over certain parts of the world, in a nutshell it is because certain people are inviting him in and giving him access to ruler certain parts of the world. God gave the earth to mankind in the very beginning, in Gen 1:28 God told Adam and Eve to 'subdue the earth'. It is our rightful inheritance to rule not just the seen realm but also the heavenly places where for too long spiritual forces of wickedness have had free reign.
As children of God we have the right, thanks to Christ's Resurrection and victory over Satan, to decree and declare for God's forces to subdue, nullify and bring down the enemies strongholds in the unseen places. In John 16:33 Christ said, "These things I have spoken to you, so that in Me you may have peace. In the world you have tribulation, but take courage; I have overcome the world.” If there was ever any doubt as to whether Satan is supposed to be in control of the world right now here is the answer. Yes The Great Tribulation will come, yes Christians will be and are being persecuted, but the idea that God's wrath is already here is misinformation, it comes from Satan and his host to prevent us from bringing down strongholds in the heavenly places whilst we are still on this earth. There is no reason to just accept 'that's the way it is' because Jesus overcame the world (and Satan) through His sacrifice and resurrection.
To conclude, I hope some of these words have resonated with you and ultimately I'm only trying to share the message of hope Christ gave us so that we can build up the body of Christ in the time that remains. We are not called to be pessimists and those seeking after dissension amongst our own family, we are called to be Christ-like and this is best done through casting out fear through perfect love.