The "world" that God commands us to be separated from is not the created things of this earth, but rather all that is under the influence and control of Satan and unregenerate human beings. This world system and all its means of influence over mankind is under the control of the devil. The only way to be a child of God is to forsake this present, evil world and become a citizen of God's kingdom. We then live as a pilgrims and strangers here. If we are a friend of this world the Bible tells us we are an enemy of God. If we love this world then the love of the Father is not in us. But "Whatsoever is born of God overcometh the world." So how we "fit in" and deal with the world around us is very accurate evidence of whether we are truly born again or not.