The Mission of the Church is the context in which we live the Christian life, for the Church is missionary by nature; she is called at all times to proclaim the Gospel, and her attitude must always be humble but courageous. And so today, an enormous ammount of work is needed on the the part of the Church, in particular the lay apostolate.
Christ and the Church are inseparable. There is no Christ without the Incarnation, and the Incarnation is prolonged in history in the Church. The Church exists then, because Christ belongs to the history of all humanity. God's plan has always been to unite and to transform all humanity in Christ His Son.
Today's "secular" world intends to live as if separated from God, yet this same world is called to fulfilment in Christ. The Church is called to pave the way for this "secular" world to receive its glory from God.
(I took the above photo while driving in Sydney a couple of years ago.)