Those are very good questions! We will start with John 12:31, “Now is the judgment of this world; now the RULER OF THIS WORLD will be cast out” (NKJV). I believe the Lord calls the Devil “the ruler of this world” because when Adam sinned in the Garden of Eden he was, in effect, OBEYING THE DEVIL instead of God. God had commanded Adam NOT to eat of the tree of the knowledge of good and evil (Genesis 2:15-17) and the Devil (through the serpent) tempted the woman, and Adam through the woman, to eat of the forbidden fruit (Genesis 3:1-6). The moment they ate of the tree they became sinners in bondage to the Devil and were, in essence, giving their allegiance to him. From that moment on the Devil became “the ruler of this world.” 

This solemn truth is confirmed in 1st John 5:19, “We know that we (believers) are of God, and the whole world (of unbelievers) lies under the sway of the wicked one.” The Devil’s influence over unsaved men and women is seen in every realm of society. He uses the “things of the world” to entice men to sin and to keep them in bondage to him. We read of this in Ephesians 2:2-3, “in which you once walked according to the course of this world, according to THE PRINCE OF THE POWER OF THE AIR, THE SPIRIT WHO NOW WORKS IN THE SONS OF DISOBEDIENCE, among whom also we all once conducted ourselves in the lusts of the flesh and of the mind, and were by nature children of wrath, just as others.” We see in this passage another name for the Devil, “the prince of the power of the air.” He is a “prince” because he is the “prince of demons” (see Matthew 12:22-27) who now works in the hearts of all who have never obeyed the gospel of Jesus Christ (see John 8:44; 1st John 3:8). We also learn here that he lives in the “air” right above the earth. He is NOT in hell, as some imagine, but is free to live in the earth’s atmosphere and to exert his influence over men. Thankfully he was defeated at the cross (as we saw in John 12:31) and will soon be cast down to earth, then to the bottomless pit, and finally to hell (Revelation 12:9; 20:1-3 & 20:12).

In 2nd Corinthians 4:3-4 we read, “But even if our gospel is veiled, it is veiled to those who are perishing, whose minds THE GOD OF THIS AGE HAS BLINDED, who do not believe, lest the light of the gospel of the glory of Christ, who is the image of God, should shine on them.” This name (the “god of this age”) also carries with it the thought of him being “the ruler of the world,” but it also draws attention to the fact that his dominion over man is limited to unbelievers for a specific time; namely “this present age.” In previous verses where we looked at Satan’s influence over men, it had to do especially with “influencing them to sin.” In our present passage his influence is “to blind the eyes of men to the gospel.” He does NOT want them to see God’s glory revealed through His Son Jesus Christ so he introduces false doctrines which blind men to their need of Christ, to who Christ is, and to what Christ has done to save them. When he is successful in blinding them to the “light of the gospel,” they are kept in bondage to him and have unwittingly chosen to make the devil their “god” instead of bowing to Jesus Christ, who is “God manifest in the flesh” (see 1st Timothy 3:16).  (377.5)  (DO)