How can we finish the task? What is the role of the church? How can churches and agencies better work together? These three questions are more intertwined than we might at first think. I want to suggest three simple ideas which every church can work on, and which I theorize (really, believe firmly) can lead directly to sustainable closure.
First, work on making the church resilient. Any given church needs to work on its endurance. "Resilience" can be defined as "able to recoil or spring back into shape after bending, stretching or being compressed; able to withstand or recover quickly from difficult conditions." Nassim Nicholas Taleb has written a lot on this (and much of what he's written, I don't yet fully comprehend!). His basic idea is that in a complex world much of the future cannot be predicted. The things that impact the most are the 'black swans' or 'outliers' - the things we could not have anticipated. Thus, since we cannot prepare for a specific future (because we cannot predict it), the way to prepare is to become "anti fragile" - to become more resilient. I think there's much in this idea for any church. Viruses spread based in part on how easily communicated they are (stickiness) and with how long they endure in the agent. A disease that is 100% communicable (spreads to anyone who comes into contact with it) yet only lasts 1 hour in a human host will (probably) not spread as far as a disease that is 10% communicable yet lasts for a month. Making the church resilient means increasing the length of time that it has contact with the people around it. Measure efforts to achieve resilience by how long the church lasts.
Second, work on making the church 'stickier.' This is probably not the best phrase, but it's the other half of the equation hinted at earlier. We need to help the church last longer among a community, but it's of little importance if it has no impact on the community. Rodney Stark has noted that nearly all conversions happen in the context of relationship, yet studies show 86% of Muslims, Hindus and Buddhists (all non-Christians, really) do not know a believer (and maybe more). Any individual church needs to work on changing this statistic--to increase the number of unbelievers that church members know, and to increase the stickiness or virality of their witness. Another way to put this is to make the church a bigger blessing to the community it is in the midst of. Measure efforts to achieve stickiness by what percentage of believers know unbelievers, and how many new unbelievers have joined the church in the past year (as a ratio to believers).
Third, work on making the church better at impacting the fringe. Every church has a domain of influence. Churches are made up of people, and those people of spheres of relationships. This "boundary of influence" can be mapped. Generally speaking, the church's influence is going to probably wane at about 2 or 3 steps from the core of the church.
For example, my mother-in-law doesn't live in my town. She doesn't go to my church except when she's visiting. I might share with her a particularly meaningful sermon or thought I got from church today: this is not as probable as my hearing it. It's even less probable that she will pass it on. From the church to me is one step; from me to my mother in law is 2 steps; beyond my mother in law, is a very low probability 3rd step. Obviously, some famous churches reach far into the 3rd step. But the vast majority of churches do not. There is a "fringe" or boundary to their influence.
To go back to the negative analogy of a virus, we don't worry too much about the common cold. It's not particularly destructive, even though it's a nuisance. What we worry about are diseases that can spread easily--and can be transmitted to far places by a plane. That's because those kinds of diseases are resilient, sticky, and can impact the fringe. They can make the long jump to new places. Making the jump is important if something is to spread.
It's important to the Gospel if we are to reach the ends of the earth. If you could map all of the churches in the world, and their boundaries, the "unevangelized" or "unreached" would be clear -- those people who are outside the boundary of influence of any church. The way to deal with the unevangelized is to get better at extending beyond the boundaries.
There are two ways to do this. One way is to get more famous, more well known, so that the extent of your influence is bigger. I don't think that's valid, because "fame is fleeting." When people die, their influence is lost. Fame is not resilient. People have heard of John F. Kennedy, Michael Jackson, Marilyn Monroe, Pope John Paul II, Mother Theresa - but while we think of them as models (some good, some not so good), they for the most part do not shape actions.
Another way--and, I think, a better way--is to get better at "planting" new churches just where your influence ends. This is akin to taking the fire you have, and blowing the sparks over your fence to your neighbor, so he can have some fire as well. Then, he can pass it to his neighbors. Measure efforts to make the long jump by number of believers or Bible studies attended by people who do not attend your main worship service.
If every church were better at "jumping the fringe" then maybe agencies wouldn't be needed. But not every church is, and there's lot of unevangelized places and individuals who are dying without the good news. So, people who are passionate about the unevangelized have created agencies--missionary teams that have gotten very good at doing "long jumps" into lightless terrain. But even still those agencies can't tackle the job on their own. So, they're trying to get very good at starting churches that impact the fringe as well. I think these are the three most important things a church needs to do (and wound in these is much of the DNA of the Gospel). If we could do these things really well, we would see an explosion of light rippling across the planet.