I can't tell you for sure why this congregation has grown by 40% in the past 7 years. One thing about congregations, you just can't do controlled experiments on them. But their history goes something like this: 1949-60: slow and steady growth to about 200. 1960-1965: Rapid Growth to about 500 (not so rare in those days!) 1965-1988 Plateau/slide to 400 (considering that many of our congregations dropped by half in the late 1960's, this is not as bad as it sounds.) 1988-1992: growth spurt to over 500. 1992-2000, 500-550, which is a glass ceiling hard to burst through. 2001-2007: 489-720.
One thing we did in 2002 was add a second minister to our staff. This was a huge task which was paid for with every penny the congregation had in savings and aggressive fund raising for three years. In some ways a second minister is more expensive than a first minister, because in order to attract someone (outside of a Seminary city, anyway) you have to be paying everyone better than you probably are, you might need new office space, and so on. It's also a huge task because nobody will help you with it. A 100 member church looking for its first minister has everyone's sympathy. There used to be an entire extension program devoted to helping churches hire their first minister, and more than one chalice-lighter grant has helped a congregation over that hump. If they took that minister and grew by 50 members they were considered very successful. But the 400 member church trying to hire its second minister has nobody's sympathy. I said more than once in that near decade of plateau that if I could only have an extension grant for a second minister I could add a small congregation's worth of members to my congregation. I was wrong. With a second minister, we added a medium-sized congregation worth of members.
Other things were right. 9/11 happened. The senior minister (me) quit getting sick and needing months of recuperation time. In our case, a second minister not only did all the things a second minister usually does but changed some very unhealthy dynamics in the church about which I had no leverage by myself. Our city grew almost as fast as the church. Still, if I had to suggest one intervention for a 400 member church which was willing to become a large congregation, that's what I'd suggest.