What is the mission of the church? Hans Urs von Balthasar has a fascinating excerpt from his Theo-Drama IV, in which he tries to explain.
Essentially, his answer is that the church’s mission is meant to redeem and transform the culture. von Balthasar says,
The whole notion of “Church”, in radical contrast to the Synagogue, is centrifugal; not only is the Church open to the world, which in principle already belongs to Christ: she is also jointly responsible for it. It is not enough to preach the message of salvation to the world from outside: this message must permeate it like leaven, becoming disseminated throughout it. What we are speaking of is—in the modern expression —“inculturation”. The cultural materials that exist in the world must be taken up and adapted, albeit critically. Again, this must not be done by the forced imposition of Christianity onto a reluctant substratum; conquests of this sort continue to take their revenge centuries later. Rather, there must be a loving appreciation of the existing values; it must calmly be shown that they are genuinely fulfilled only in the message of Christ.
However, this is difficult, from both sides. It is difficult on the part of the culture that is to be transformed: particularly in its most highly developed form, its Gestalt exhibits an earthly perfection, like a work of art; in its own order, it seems incapable of improvement. The Christian reality, however, lives entirely in relationships of continual transcendence and in principle (not accidentally) breaks open the complacent, earthly forms, putting them in touch with a Catholic universality: thus they must open up to the world around them but also to the world above them. It is even more difficult on the part of the Church; the gospel message is embedded in the structures of the “missionized” culture, a fact that threatens to bring the movement of the missionary Church to a full stop. Inculturation threatens to adapt Christianity to the existing culture; the salt of the gospel is in danger of losing its savor. Amalgams are formed between Christianity and secular culture; at times this produces marvelous cathedrals of art, of philosophy and of piety, yet it is not clear whether these are a pure expression of the gospel.
The Church can only effectively pursue her task, therefore, if she herself alternates between two impossible poles: preaching to the world purely from without and transforming it purely from within. As Church, she must penetrate without becoming “establishment” and advance without leaving unfinished business behind. Paul represents a kind of ideal: he founds communities, moves on, then returns to his foundations, but without finally settling down there. The profile he presents should be that of every community and of each individual: the Church must put down roots where she is, yet without coming under the spell of the place. Here again we find that this paradox, which is baffling at an earthly level, reflects the discipleship of Christ, who comes into the world and leaves it (Jn 16:28) and yet stays with it until the end (Mt 28:20); for it is in his exodos (Lk 9:31)—which is his Eucharist—that he is killed and so remains with us. (Theo-Drama: Theological Dramatic Theory, Vol. 4: The Action, section IV, C, 3)
So then, the church is given the mission of fulfilling, redeeming, deifying the culture. And so the church must go into the culture, and adapt and fulfill it in Christ.
Balthasar is certainly right in saying that in doing this, the church must ride the line between simply evangelizing “from the outside” only (ie, never entering into the culture, but only being a culture in and of itself) and being “institutionalized” or adapted by the culture.
Rather, the church must go into the culture, appreciate it, and then elevate it in the gospel. In other words, Christianity wants to take culture and take it up into God’s own life so that it can participate in his own glory.
This reminds me of a line by Robert Barron, that God is not “a competitor with his creation”. Rather, God’s intention in the gospel is to elevate, redeem, deify creation — and culture, and art, and music, and family et al — through Christ’s death and resurrection. And in this way, as Barron says so aptly, the gospel makes Christianity “the greatest humanism that has ever appeared”. This is the church’s mission.