If any man love the world, the love of the Father is NOT in him

1 John 2:15-17
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.
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.
17 And the world passeth away, and the lust thereof: but he that doeth the will of God abideth for ever.

We cannot live double lives! "No man can serve two masters: for either he will hate the one, and love the other; or else he will hold to the one, and despise the other. Ye cannot serve God and mammon." (Matthew 6: 24)
We can live IN the world but should never live OF the world! As soon as we decide to live of the world we are no longer serving God. Even though many people think they can have one foot on the world's side and the other foot on God's side, this is not true. I really like how in verse 17 of 1 John it says those that doeth the will of God will abide forever. The world will not be there to back us up in the end, only God will, IF we choose to live our lives following Him.