Sunday School
Those of us who came to Christ (especially within the evangelical movement) in the 70’s and 80’s and before are very familiar with the notion of Adult Sunday School. We got much of our deeper teaching in this structure, usually either directly before or directly after our Sunday worship service. In the churches I attended in the 80’s and 90’s we had age based communities, that were rather larger (10-50 people) subgroups of the church, where people who had things in common could learn together. Usually there was a single teacher, or a rotation of teachers (opportunity for new teachers to develop) and a couple of administrative types (organize socials, coordinate teaching schedules, deal with coffee and treats, etc). The social life of the church was somewhat organized through these groups, who generally formed as new married couples, or new parent groups – who helped each other and grew deep friendships along the way. Kids grew up together. Often there were bespoke college age and adult singles classes, because they were different adult communities with different synergies that didn’t really fit well in the married adult communities.

Because all the groups met on Sunday morning, in a classroom at the church, a significant amount of facility investment was required to support this program/method. I remember churches with large multi-purpose rooms with multiple dividers that could become 4 or 6 classrooms or more on Sunday morning being common. Typically the classes were space constrained, and so could not grow because of classroom space. Also typically, the middle age group (parents 35-55 with kids who were 10 – 30) was the largest class, as younger classes were more transient, and subject to kids issues, and older groups started to dwindle. Most church leaders and core members were in this group, as they had more resources and time to dedicate to ministry.

One thing that happened frequently, was that these groups broke into subgroups, based on whatever, but to newcomers, they might be perceived as cliques within the larger sunday school class and therefore barriers to entry for new people. I remember at one church I attended, there was a group that the senior pastor attended, and it was that middle age group. Almost all of the elders and deacons were in that class as well, and that class had the best rotation of teachers in a church that really focused on deep bible teaching. At some point it was recognized that the group itself was an obstacle to new members plugging in because of the length of time the group had been together and the cliques that had formed. A formal decision was made to break up the class and re-form some smaller groups. There was lots of drama around this.

Because of cliqishness, real-estate, schedule sunday school classes cannot grow very fast, but the administration is easy. One advantage is that often childcare is guaranteed – this can be a boon for parents, using the church’s child-care facilities. One other advantage is that they are more “invitational” – you don’t have to check with the leader before you invite friends and neighbors.

Small Groups
I have experienced small groups in every church I have attended for very long. They typically meet in homes, are usually a mix between teaching, social, and emotional/spiritual support and accountability. Small groups are usually between 3 and 12 people and can be demographically homogeneous or heterogeneous. Small groups usually have a host and a leader who sometimes is the same person. The administration of the small group program is typically centralized, and fairly flat – that there is a volunteer or vocational leader who has one or more administrative helpers that help find and create small groups, maintain a catalog of groups, and recommend/distribute small group study materials. In order for groups to grow, a matching function for new people who want to join small groups is necessary – to help them find a group that they will “fit” in.

Because the bar for leadership can be highly variable, and hosting is virtually none, often the pool of potential small group leaders is large. Since there are fewer infrastructure requirements, small groups can grow fast, and huge, with some churches representing that they have more people in small groups than actually attend the church.

Small groups have different problems than the cliques that form in the sunday school classes. Small groups can be very transient, lasting from 6-12 weeks to many years. Once a small group has been together for 2 years in the same structure it is hard for new people to “break through” to get really deep. Small groups collapse when a leader moves or gets burned out, and small groups can dwindle over time. People can form small groups outside the context of the congregational program with different agendas and different programming. Each group tends to decide how social, teaching, prayer and care/support are integrated into the group. Within any church small group program, groups can be focused on any of these aspects or completely balanced across them, there is little consistency between groups. This lack of consistency is both a strength and a weakness – that is, unless the groups are catalogued by these attributes (fairly uncommon) a potential new member has no good way of knowing what they are getting into. Small groups also have a geography problem that Sunday School does not. Many churches (especially growing dynamic churches) pull from a broad area, compromising the convenience of small group participation for those who live farthest out.

Some churches try to do both
I have been at at least one church that has tried to maintain a sunday school program while forming small groups. I think that this is not sustainable, as it requires all the resources to support both programs. Some people will focus on one or the other, and some people will participate in both, but I believe that neither will flourish, because their purpose and focus are different and to engage on either expensive. A double minded man…

Conversion
Moving from Sunday School to small groups can be a trial, and divisive. The existing Sunday School teachers an administrators need to be be engaged to become small group leaders to generate enough choices to grow. If the church is going to push content into the groups (follow the sermon series, follow a cirriculum) that needs to get support from many leaders and teachers. Facilities may need to be re-purposed (classrooms will be empty) to avoid waste.
Really, the whole congregation needs to understand why the conversion is necessary and support the “goals” of the conversion.

Why convert?
Really, it is because you want to trade to a more scalable model. If your church is growing and the Sunday School program is not, then often small groups are seen as a way to get those new people involved more than just on Sunday morning. Churches that have outgrown their facility and are looking at multi-services also look to smash sunday school programs, as those often would be replaced with a service. I have seen/heard of churches that did two services with alternating adult Sunday School concurrent with each service. It’s scales, but not really beyond two services.

Conclusion
One thing that is really clear in my mind is that small churches do not use small groups very often. Perhaps because the community is so “close”, that they don’t need to create “artificial” communities within the body. There is an inflection point around congregation size somewhere between 100 and 500 where small groups start to really have an advantage in scalability. You need to know what your groups are for, what their purpose is. Is it outreach, community, discipleship, or some other thing? You also need to know how your groups are catalogued; whether geographically, by purpose, by demographics, or some other criteria, this is critical for matching people to groups.

I think that some churches whose membership is stagnant or dwindling have implemented small group programs as a way to reduce the transience in new attendees. Leadership may feel that this is a quick fix to solve the back door problem (people dribbling away). I think that this is a flawed strategem. If people are leaving, you should find out why and fix that before you try to make superficial changes, because when those also fail, it is one more disappointment for leadership and one more costly program.