In times of exile, Jeremiah tells us to get married and have children, and have our children get married and have children too! What does this mean? The key to this is in the final thought, "that you may be increased there, and not diminished." To apply this spiritually, it means we should try to increase our numbers— to grow.
Personally, I have done my part the physical way: Beth and I have had three children in this church. Others have married someone out of the world, who have become converted members of God's church. This is not the normal way it should be done, but it happens every once in awhile. If God is working, if He is calling that person, He finds a way.
This piece of advice deals with "going to the world." The earlier commands relate to "feeding the flock." The third point, "increase as you are able," suggests increase by marrying, by having children, even by some form of proselytizing. But, always, the point is conversion. It is not to happen just to add numbers. We speak of God's flock, whose hallmark is quality, not quantity.
It is very hard both to feed the flock and go to the world. The indication here is first to get in good spiritual condition, and then, if possible, increase our numbers. Matthew 6:24 says we cannot serve God and mammon. It is dificult to do two things at once well, so we have to choose which is the most important. If Jeremiah 29:5-6 are any indication, the first and most important thing to do is to get oneself straight with God, and what resources are left over can go toward increasing one's numbers.
The first two points are most important right now, because God sent the church into exile because of sin. We must get rid of the sin first. Once we solve the problems, we will have the spiritual resources to increase our numbers. In Mark 10:28-30, to pull the principle there out of context, Peter says, "Master, we've left all to follow You." And He says, "Don't worry, Peter, for whatever you have lost I will return to you: mothers and brothers and sisters and fathers a hundredfold." So He will increase us. It is just a matter of when.
Richard T. Ritenbaugh
How to Survive Exile