The book of 2 Corinthians 5:20 says that we are ambassadors for Christ. 
Like any other ambassador here on earth posted to his station country, we are also ambassadors (representatives) of Christ to reconcile the world back to Him.
We are posted into the world though we are not of the world. 
We are to win the world to Christ.
In doing this we give people’s lives a meaning- true meaning in God.