Satan is the god of this world. But that does not mean he is in any way equal to the one true God. And the world is a mixed bag. Because we are all fallen and sinful, our world is fallen and sinful, and Satan exploits that to the max. But still, the world is God’s world. He made it and pronounced it good. Although tarnished and stained now, it will one day be recreated for us to enjoy forever.
Christians do not believe in metaphysical dualism. That is, we do not believe there are two equal and two eternal spiritual forces in the universe. There is just one living God – end of story. Satan happens to be a created being. He is not omnipotent and so on.
And we believe that while God allows him to do some dirty deeds on planet earth, all this is limited. Just as Satan was allowed to torment Job only as much as God allowed or permitted (see Job 1:12 and 2:6 eg.), so too in other areas. God has Satan on a leash in other words. And that is good news indeed.
God is on the throne – not Satan. God is working out his purposes – not Satan. God is sovereign – not Satan. But yes, Satan and the demonic hordes can do great damage indeed, which is why we must always have on our spiritual armour (as in Ephesians 6:10-20, eg.), and why we must keep praying and engaging in spiritual warfare. Let me offer a quote from the commentary by George Guthrie on this:
In calling Satan a “god,” the apostle does not ascribe divine status to the evil one but rather speaks of the functional status given him and the subordination of the fallen world to him. The phrase is comparable to John’s “ruler of this world” (John 12:31; 14:30; 16:11). Ever since the fall, this “god” has beclouded the Word of God (Gen. 3:1), blinding the minds (1 John 2:11) of those who do not believe. Thus peoples’ minds constitute one very significant battleground in the cosmic conflict between God’s gospel and the twisted machinations of the god of this world (2 Cor. 2:11; 3:14; 4:4; 10:5; 11:3).
And Judith Diehl stresses the limits of Satan’s ‘rule’:
The “god of this age” is a unique phrase, as it occurs only here in the NT. It refers to Satan and the dominion that he has over those who reject God and his agent, Jesus Christ (see 2:10–11). Here, Satan represents lawlessness, darkness, unbelief, moral depravity, and the worship of idols (see 6:16, especially apparent in polytheistic Corinth). Paul is fully aware of Satan as his personal adversary and Satan’s attempts to impede Paul’s mission and his unique calling to ministry. We can notice, too, that in Paul’s view, the power and influence of Satan are limited to “this age.”
See here for more on the reality of Satan.
And see here for more on the need for spiritual warfare and protection.
Consider also the matter of the world and our involvement in it. When we read about how Satan is the god of this world, we need to be clear on just what is being said. The Greek word for world – cosmos – is used in different ways in the New Testament. Often it can just mean the globe that we all happen to inhabit.
But at other times it refers to the current evil system that we must have nothing to do with. Thus, we all LIVE in this world, but believers are not to take part in the evil, worldly system that is all around us. That we can do both simultaneously is made clear by Paul when he said this in another context:
“I wrote to you in my letter not to associate with sexually immoral people—not at all meaning the sexually immoral of this world, or the greedy and swindlers, or idolaters, since then you would need to go out of the world” (1 Corinthians 5:9-10).