David Wells described worldliness as
that system of values, in any given age, which has at its center our fallen human perspective, which displaces God and his truth from the world, and which makes sin look normal and righteousness seem strange. It thus gives great plausibility to what is morally wrong and, for that reason, makes what is wrong seem normal. (Losing our Virtue, p. 4)
See 1 John 2:15-17; Romans 12:1-2.