'Worldliness' is a new book edited by C. J. Mahaney. The book is a very good read, and helpful on biblically defining worldliness and teaching readers the cure for it in Christ Jesus.
Here is a snapshot of the first chapter on 'Worldliness':
Worldliness is not merely external threats from outside of the Christian; worldliness come from the heart (what the heart loves, 1 John 2:16: cravings, lusting, boasting are three things specifically mentioned that come from the heart).
ESV 1 John 2:15-17: Do not love the world or the things in the world. If anyone loves the world, the love of the Father is not in him. 16 For all that is in the world - the desires of the flesh and the desires of the eyes and pride in possessions - is not from the Father but is from the world. 17 And the world is passing away along with its desires, but whoever does the will of God abides forever.
An excellent definition of worldliness is defined by Pastor Ian Murray:
Murray: "Worldliness is departing from God. It is a man-centered way of thinking...It judges the importance of things by present and material results; it weighs success by numbers; it covets human esteem and wants no unpopularity; it knows no truth for which it is worth suffering...It adopts idols and is at war with God."
The root issue of worldliness is within us. We must not define ourselves by, or boast in, anything we possess and/or accomplish in this world. Instead, we should identify with Christ and his definition of greatness: the humble, the servant, the last.
John Owen described our worldly cravings as "living affections to dying things" (from 1 John 2:17)
The things of this world- - when compared to pleasing God and eternal life, when informed by an external perspective that is Godward- -will be exposed as being worthless. But there is a great future in godliness, and for all who do the will of God. They, by contrast, will live forever.
John Owen wrote: "When someone sets his affections upon the cross and the love of Christ, he crucifies the world as a dead and undesirable thing. the baits of sin lose their attractiveness and disappear. Fill your affections with the cross of Christ and you will find no room for sin."
Worldliness is so serious because Christ is so glorious!