The simple answer is found in II Corinthians 4:3-4. It says - **"But even if our gospel is veiled, it is veiled to those who are perishing, [4] 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." **
Now, let's see Isaiah 25:7 - **"And He will destroy on this mountain The surface of the covering cast over all people, And the veil that is spread over all nations." **
Note that the focus is "the people" (mankind) - the devil is too small for both the Lord and we that are fighting on His side.
When the devil was thrown down the earth after rebelling against God, the Bible says one of the immediate things he did was to start weakening the nations. In other words, he embarked on the enterprise of luring the earth and its inhabitants to turn their back against the perfect Will and Plan of the Lord. One of the core ways to achieve this was to cast a spell of covering upon the earth. It's just like blocking something from light so it will be dark.
So, in Genesis 1:2, it said darkness was upon the earth, the face of the deep. It was not as if light ceased, as the Holy Spirit who was hovering is a Carrier of light. But something was in between the light and the earth, blocking the light from shining upon the earth. So, there is a covering cast over all people, And a veil that is spread over all nations."
In 2 Corinthians 4:3, Apostle Paul was talking about people who are perishing because the Gospel of Christ that brings salvation is still veiled to them. In vs 4, he now explained why they are rejecting the Gospel of Christ that is meant to save them from darkness: "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." For these unbelievers, it's not solely their fault that they are still living the way they are living. There is a spell cast upon them that blocks them from the truth.
The difference between them and you who has received Jesus as Lord and Savior was that at some point, the covering was removed over you,and this explains why you could see and embrace Jesus. It was the point where the Holy Spirit took time to hover upon you to break the spell upon you - so when the Word would eventually come, it was easy to be accepted.
Now, as a warrior king, the Lord expects you to be a frequent occupant of the realm of the spirit so you can join in the battle of helping cast the veil off as many people as possible. This is possible through priesthood, particularly intercession.
One of the major things we do on the platform of intercession is to win the souls of people perishing. If we don't do this, all of our evangelism are just a waste. So, those unbelievers around you, take them to God via the incense of intercession as you engage priesthood, and you will be able to partner with the Lord to destroy the covering cast over them, and the veil that is spread over them. You can do this for specific cities, countries, tribes, continents, etc.
Now, when you do this, what happens next? It's right there in Isaiah 25:8 - "He will swallow up death forever, And the Lord God will wipe away tears from all faces; The rebuke of His people He will take away from all the earth; For the Lord has spoken."
So, God, after the veil has been destroyed and the sinner have turned to the light of Christ's salvation, will now "swallow up death forever". What does this implies? Remember that the unbeliever living life without God is dead - we are talking about spiritual death here.
Anyone still living in rebellion against God actually has a veil over them, and the person is dead because they are separated from their Source. It's just a matter of time, other lesser forms of deaths will set in, including the death that will transport the person to hell. But after the covering cast over them has been removed and they have turned to the Lord, then He will swallow up the death they deserve.
When an unbeliever that has been living a dangerous life like smoking and alcoholism which will get them killed prematurely, now repented and stopped those things, God has swallowed up death over their lives. They will now have fellowship with God and will no longer be separated from Him..