Why are so many churches of Christ so small?
Why do so many of them seem to have so few young people?
Why do so many of them have so many problems?
Why are there so many churches? (denominations)
What immediately strikes me is the focus of these questions. They are all internal: the work is too hard, we aren't making enough progress, we're losing ground. In essence, that is the nature and cause of the problem. We aren't looking at the great and rapidly increasing need for the gospel, we are only looking at ourselves. "For we dare not class ourselves or compare ourselves with those who commend themselves. But they, measuring themselves by themselves, and comparing themselves among themselves, are not wise" (II Corinthians 10:12).
The second thing is that there is a perception problem. Suppose I ask "Why are there more people in the world who are under the age of 10 than there are over the age of 90?" Most would think this is obvious, people die. Not everyone manages to live to a very old age, despite our many medical advances. Does it indicate a problem? No, it only states a fact of life.
Why are so many churches of Christ so small? Because most churches are relatively young. In the overall scheme of things, a majority of congregations do not survive beyond three generations. Why? The first generation is fired up, excited, and willing to work at almost any cost to get the gospel spread. The second generation sees the "dragon" slain, and instead focuses on maintaining what was won. The third generation, eager to make its own mark in the world, wants to make changes and often those changes are toward error. And so, over time, congregations fade away or fall away only to be replaced by a new congregation many years later. Years ago I heard a lesson on this typical trend and wrote up my own version of it. See "The Three Chairs" for more details.
In a survey done of all churches in the United States, it was found that "Half of all congregations have fewer than 100 regularly participating adults, and one-fourth have fewer than 50. One in 10 have more than 1,000 adult participants." [Distribution of Church Sizes] Seen in this light the churches of Christ are not that much different. A survey done of all churches of Christ in 2006, which ignored particular beliefs, found that 29% of the congregations have 100 or more members. 42% of the congregations are under 50 members. So yes churches of Christ tend to be a bit smaller, but they also tend to be newer as well.
Still, we should not be surprised that faithfulness is not generally popular. "Enter by the narrow gate; for wide is the gate and broad is the way that leads to destruction, and there are many who go in by it. Because narrow is the gate and difficult is the way which leads to life, and there are few who find it" (Matthew 7:13-14). The road to heaven is singular and narrowly defined. People have a tendency to do as they see fit. Therefore, there will always be fewer who are willing to stay on the straight and narrow.