The Bible says in Matthew 6:24 that the top competitor for our commitment to Jesus is money. Jesus doesn't want our money, He wants our heart. Giving allows us to demonstrate that He is more important to us than material things. He gives us all things, do we trust Him enough to give some back? Giving shows our faith that He is in control. God can do more with what we have then we could ever do. Remember what He did with 2 fish and 5 loaves of bread?
There is no organization with more potential to change the world than the local church!