(For the sake of argument I define purpose as the reason for our existence. And I am defining mission as our unique way of fulfilling our purpose.)
Paul made these points when he was preaching in Athens to the Epicurean and Stoic philosophers.
- God made everything.
- God doesn't need anything.
- God made every nation of people (from one man).
- God intended that the whole earth be inhabited.
- God predetermined where each person would live, their lifespan, and when they will come into existence.
- And because of this, God expects everyone to desire to know him personally.
- Therefore, our primary purpose is to know him (who is close) and to make him known.
It seems to me that our purpose isn't contained in a need God has. In other words, God facilitates different missions for each of us, that contribute to God's overall plans. But none of us is needed by him for his plans to be fulfilled. He invites us to closeness with him by working along side of us in our unique gifting, location, and culture. We are successful as long as we are being who he planned for us to be, and doing what he planned for us to do (Ephesians 2:8-10).
This is why each Christian's mission is different (in approach and ability), yet our purpose is the same (in message and for whom). It makes sense, then, that success is determined by how well we use our unique gifts, abilities, and time in fulfilling our purpose and mission for God's glory. It is God who is looking for a return on his investment.