The central hypothesis of the limited enthusiasm model is that conversion growth in the church is driven by a sub-group of church members called enthusiasts. Some new converts become new enthusiasts. The Spiritual Life model extends the limited enthusiasm model by allowing enthusiasts to generate spiritual life in the church, which can, in turn, improve their effectiveness as agents of conversion. The analogy is with the spread of a disease, where the enthusiasts are “infected” believers passing the faith on to unbelievers who catch the “disease” of religion. In this model, the infectiousness of the “religious disease” can vary.

The following model details supplement the Spiritual Life Model Overview.


The model consists of three groups of people: unbelievers, enthusiasts who alone are responsible for spreading the faith, and inactive believers. If demographic effects are included, then unbelievers are split into those open to conversion and those hardened against it.

The spiritual life model is built on the Limited Enthusiasm and the Demographics models, which give fuller details of these assumptions.

There is an additional variable called Spiritual Life. This is a measure of the level of spiritual life in the enthusiasts, i.e. the spiritual life in each individual averaged over the whole group. Spiritual life will include activities in prayer, worship, discipleship and teaching, as well as internal spiritual inclinations and feelings that may drive behaviour. There is no attempt to separate these quantities out or measure them in terms of quantity or quality. Spiritual life is like a “spiritual temperature” that measures how hot the church is in terms of the things that matter most in the spiritual side of its beliefs.

Spiritual life is related to the sociologists’ concept of spiritual capital which is a spiritual version of social capital: the collective benefits of individuals working together, i.e. networks have more value than just the sum of the individuals. In the case of spiritual capital, the church has more spiritual value than that of the individuals alone, because by interacting in a network, there are greater spiritual benefits to members. Spiritual capital is more specific than religious capital, as the latter may include rituals and acts with little or no spiritual benefit.

Spiritual life is an example of a “soft variable”, one that can be understood conceptually but is hard to measure. They are often measured subjectively by comparison with past estimates. Thus, more people praying, or running Bible studies, will make the church a more “spiritual” place than it was before. More spiritual life will mean that the church has more to offer people seeking God and something different to offer from non-religious organisations and society; thus, they could see more converts. Spiritual life is thus understood in terms of what produces it and how it affects other things, both of which are easier to measure. For that reason, it is given a maximum value, which can be taken as 100%. Thus, spiritual life will be harder to achieve as it gets closer to that maximum.

Dynamic Hypotheses

The dynamic hypotheses are:

Conversion Through Enthusiasts (R)Enthusiasts are responsible for spreading the faith, i.e. conversion to the church. The more they convert, the more enthusiasts. This accelerates growth. Spreading the faith can be by numerous means.
Loss of Enthusiasm (B2)
After a period of time, enthusiasts lose their potential to convert. This slows and limits growth.
Diminishing Susceptible Pool (B1)As people are converted the effectiveness of the enthusiasts on the remaining unbelievers become less as proportionally more of the enthusiast’s time is spent on believers. This slows growth.
Believers Leave Church (B3)Believers can leave the church and become unbelievers again, in the sense that they no longer belong to the church.
Leavers May Return to Church (R2)People who leave church may return to church but only through contact with an enthusiast. They are indistinguishable from unbelievers
Growth Enhanced Through Spiritual Life (R3)The communal action of enthusiasts generates spiritual life. This in turn enhances their effectiveness in conversion.
Spiritual Life Fades (B5)Spiritual life naturally fades in the individual and thus in the church, unless there is activity by enthusiasts to maintain it.
Spiritual Life has an Upper Limit (B4)Spiritual life has a maximum value due to the limited capacity of people. The spiritual life of the church is the average value over the enthusiasts, not the total, and is thus also limited. It will be increasingly harder to increase the closer it is to capacity.
DemographicsThere are further assumptions connected with births, deaths, leaving the church, reversion, softening and hardening of unbelievers, as in the Demographics model.

System Dynamics Model

Unbelievers convert to believers through contact with enthusiasts who have “spread the faith” to them. New converts become enthusiasts. Enthusiasts only remain active for a limited length of time before becoming inactive and taking no further part in spreading the faith. Inactive believers may leave the church and be open to “re-conversion”, i.e. rejoining the church. Enthusiasts generate spiritual life in the church, which in turn makes them more effective in conversion. See figure 1.

Figure 1

Growth is driven by enthusiasts, R1, and opposed by the diminishing pool of susceptibles, B1, as in the basic Limited Enthusiasm Model. Additionally, the impact of the enthusiasts is reduced by their limited enthusiastic period, B2. However, an additional reinforcing loop, R3, accelerates church growth as groups of enthusiasts increase in their spiritual life and hence in their effectiveness. It is this variability in the enthusiasts’ effectiveness that is the primary focus of this model.

The growth of spiritual life is opposed by that life fading in individuals, B5. As spiritual life gets closer to its maximum value, its growth is increasingly opposed by loop B4.

The loop R2 enables the model to have a long term application and illustrates generational cycles of growth that result from people leaving the church, B3, as in the Demographics Model.

What is Meant by “Spreading the Faith”?

All aspects of conversion and spreading the faith are discussed in the limited enthusiasm model.

What is Meant by Spiritual Life?

Spiritual life is a property of a single person – normally a Christian believer in the context of church growth modelling. It may also be seen as a property of a church, represented as an average group of believers acting together as one.

Spiritual life is that aspect of a believer’s life that helps them gives them a greater understanding and practice of the spiritual aspects of the Christian faith. The primary consequence of that life is that a believer is better able to pass the faith on to unbelievers. A second consequence is that the believer is better able to build up the spiritual life of the church, that is, other believers.

For more details see Spiritual Life in Dynamic Modelling.

How Do Enthusiasts Generate Spiritual Life?

There are both natural reasons, which could be true of any organisation, and also reasons that depend on God’s action. These two types of reasons overlap in their effects, thus they are presented here without any attempt to separate them.

  1. Enthusiasts are able to encourage each other and spur each other on in zeal for building a lively church and making converts:
    • They report back to each other the stories of their attempts to win people to Christ;
    • New enthusiasts bring fresh life and zeal to the group;
    • They plan together how to make a better church and have a more effective witness;
    • They help each other in their own personal faith, admonish, correct and inspire;
    • Enthusiasts are better able to hold to the fundamentals of the faith, maintaining a strong and strict church, following Kelley’s Thesis;
  2. Enthusiasts pray together, and God answers their prayers:
    • God releases his Spirit on the church, increasing every believer’s spiritual life and giving spiritual gifts;
    • God makes changes in the personal lives of the enthusiasts, making them more like Christ;
    • God releases his presence, creating a state of heaven on earth that empowers enthusiasts to take on the world;
  3. Enthusiasts are better able to build up the rest of the church because there are more of them.

Why Does Spiritual Life Enhance Growth?

  1. Believers who are more like Christ are more likely to attract people to the faith ;
  2. A church whose life is spiritual portrays God better and more likely to attract people and keep an unbeliever long enough so that they become converted; better churches have more impact;
  3. The God who moves in the hearts of believers, making them better enthusiasts, also opens the hearts of unbelievers to respond. Enthusiasts are effective because God is effective in the lives of those who witness AND those witnessed to;
  4. Enthusiasts become more active as God’s presence empowers them to take on the world;
  5. A stronger and stricter church creates a more transparent church; better able to present what it believes, and more able to win and keep those who identify with those beliefs.

Why Does Spiritual Life Fade?

Unless any belief is reinforced, its importance in a person’s life will become less. To be a Christian, a person needs to practice Christianity. This is why it is so hard to be a Christian without being a part of a church or some organised group. For beliefs to be maintained, time needs to be spent on them, or the mind’s natural tendency to forget kicks in. Spiritual life is no different. Unless there are “things happening” in a person’s life to increase the spiritual side of life, such as prayer, Bible study, interaction with others, and worship, then that life will fade by neglect. When this life is aggregated over all believers in the church, then the net result is that the church’s spiritual life fades as well. Thus, teaching programmes, evangelism, keeping worship fresh, etc., are all part of maintaining the church’s spiritual health.


All demographic aspects are discussed in the demographics model.

Model Parameters

The behaviour of the model is controlled by a number of parameters that reflect the church’s effectiveness, and the response of society:

Normal Reproduction PotentialThis is the number of unbelievers converted, and made enthusiasts, through one existing enthusiast, given the whole population are unbelievers in the absence of enhancements through spiritual life. It reflects that there is a minimum level of conversion whatever the spiritual state of the church, as God is sovereign over such circumstances.

It can also represent a natural recruitment rate by the church behaving like any other organisation. It could also mean the action of enthusiasts who, although on fire personally, their spiritual life has done nothing to enhance that of the church, perhaps because the church has chosen to ignore them and carry on “business as normal”.
Effect of Spiritual Life on ConversionHow much the normal reproduction potential is increased for a given unit of spiritual life. Essentially, the way spiritual life increases the number of converts per enthusiast, on average.
Effect of Enthusiasts on Spiritual LifeHow fast spiritual life will increase per enthusiast, given there are no barriers to that life increasing.
Duration of Enthusiastic PhaseThe average length of time an enthusiast is active in conversion before they become an inactive believer.
Reversion RateThe annual percentage loss of people from the church.
Initial Fraction of Church EnthusiastThe fraction of the church who are enthusiasts at the start of the model.
Demographic ParametersBirth and death parameters, leaving and reversion rates, hardening and softening rates. See Demographics model.

Model Results

See Spiritual Life Results.