Why does God call people to be the Church?

Is it for the upbuilding of the members of the Church?

Or does the Church exist for the sake of those who are not part of it?

Many Christians would give the first answer, but the story of Abraham's call suggests the latter. Abraham is called apart so that all the nations may be blessed by him and by his descendents. [Genesis 12:1-3] This theme continues throughout the scriptures.

It is also supported by the idea of the priesthood of all believers - an important idea from the Reformation. [I Peter 2:4-10] The function of a priest is to be a mediator between God and the people. If all Christians are priests, if we as a people have been called to be a "royal priesthood," that raises the question: priests to whom? It must be a priesthood for the rest of the world. We are to intercede with God on behalf of those who are not (yet) believers and to represent God to those who do not (yet) know him.

The metaphor of "ambassadors" which Paul uses in II Corinthians 5:20 also supports this idea, as does the reference in Hebrews to joining Christ "outside the camp" [Hebrews 13:12-13].

Clearly, the Church also exists to build up its members. It is, as has been said, "a hospital for sinners, not a club for saints." It is right for us to care for one another, build each other up, bear one another's burdens, encourage one another in prayer, worship, service, and so on. But the primary reason for the Church's existence is external. How would the Church be different if we all took that seriously?