Jesus wants us to go into the world, but He doesn't want the world to get into us. A submarine can't fulfill its purpose if it is out of the water. It must be submerged into the ocean before it can be effective. Even though the sub is surrounded by water, the water can't get inside because it is insulated. The submarine was prepared ahead of time, before it went into the sea, to have all possible areas sealed where leaks could occur.
In the same way, we are to be insulated from the world but not isolated from it. We need to be prepared ahead of time, before we go into the world, to seal off all possible areas where the world can leak in. The Lord wants us to influence the world--but not for the world to influence us. To hear the entire message on this topic go to:
http://www.churchoflivingwater.net/node/23 and click on
"Living in a Changing World"