Worldliness is our Enemy
We must remember that when Satan fell, dragging a third of the heavenly host with him, and when he succeeded in getting Adam and Eve to sin, he contaminated the world. This contamination extends to God having to remake the world one day.
1John 2:15 Love not the world, neither the things that are in the world. If any man love the world, the love of the Father is not in him. 1John 2:16 For all that is in the world, the lust of the flesh, and the lust of the eyes, and the pride of life, is not of the Father, but is of the world. 1John 2:17 And the world passeth away, and the lust thereof: but he that doeth the will of God abideth for ever.
It is pretty clear that the Bible puts worldliness (being an active enthusiast of the world system) is dynamically opposed to being a Christian. God directly commands us to be different than the world system, to stand out and apart from that world system.
This is the problem with so many neo-evangelical Christians, they are trying so hard to merge their Christianity to be a part of the world, and the world a part of their Christianity. They want the two things (the world and Christian faith) to be one. That is impossible.
In the context here, God advises us that the world is set as our enemy, because the world rejects God, because the king of this world, Satan, has rejected God. There can be no truce. There can be no cordial working together of the two. You are for one or the other. To be for one places you as an enemy of the other.