I did old-fashioned maths at school, the kind that Rishi Sunak wants us all to learn, with arithmetic, algebra, geometry – and calculus in the Sixth Form. But, as a spot of light relief in the last week of summer term in the year before my A-levels, our teacher introduced us to “new maths” – which, just a few years later, was being taught to young children in primary schools. At the heart of what we learned that week was “set theory”, a way of doing maths by grouping or placing objects together. I won’t go into this any detail, but simply say that we were shown how it could be illustrated with Venn Diagrams: you’ll know what they are even if you don’t know what they’re called. For each circle represents a set of objects. If you have more than one set, they may be quite distinct or overlap: a bigger set may even envelop a smaller one.

Why am I telling you this? Well, let’s imagine a street in Llanedeyrn. The people who live in it can be arranged into lots of different sets: by age, perhaps; or by whether they’re employed, unemployed, studying or retired; by whether they own or rent their house; or by whether their first language is English, Welsh or something else … it can all get very complicated. We can add to that organisations that they are members of: the boxing club or a reading group or a fitness class or a choir, even a group of friends who simply meet at the pub once a week (or who did before it closed). So our residents are all part of several sets of people: but how much do those sets overlap and intersect, to what extent do folk feel that they are part of a community? Do they prefer to get on with their lives more-or-less alone, or is the sense of being part of a community something for which they are yearning?

It’s clear from the passage we read earlier that the first church, in the heady days after Pentecost, was a community – a community held together by a common faith in Jesus and given energy by the Holy Spirit. Its members knew that they were part of something new and vibrant, although they didn’t quite know what; in mocking and hostile surroundings (after all, look what had been done to Jesus) it was important that they stuck together and supported each other.

So let’s summarise what those first Christians actually got up to. We read that they placed themselves under the authoritative teaching of the apostles – people who had of course not only heard Jesus’ public preaching but also talked, discussed and even argued with him over three years. I’d say that no church can call itself truly Christian if it’s not a community which wants to learn about its faith.

We read too that these Christians worshipped together, meeting in a colonnade on the east side of the Temple’s outer court. As such, they functioned as a haburah or Jewish sect (for, at this time, none of them were Gentiles – that came later). One feature of such sects was that they ate together frequently, and the Christians certainly did that. Were these celebrations of the Lord’s Supper or just fellowship meals? We don’t know, but the participants must surely have been thinking of Jesus and talking about him as they are and drank.

There’s more. We’re told that these first believers, who may well have been expecting Jesus to return to earth sooner rather than later, sat lightly to their private property. They “had everything in common”, pooling or selling their possessions and dividing them up according to individual needs. However it does seem that they kept hold of their homes and businesses: perhaps they only sold assets which were surplus to requirements. It’s debatable as to whether this enthusiastic policy was a success, as we know that Paul later had to take up a collection for the Jerusalem church which had become bankrupt!

I’ve always taken the view that these first chapters of Acts show us exactly what the Church was like in its earliest days, a time when everyone was revelling in the excitement and newness of what was going on. The sense of unity was strong, the worship was fervent, the number of Christians was growing on a daily basis, the Holy Spirit was clearly at work. This was, perhaps, a honeymoon period, for we know that it wasn’t long before problems of deceitfulness and even racism started to creep in; and of course many of Paul’s letters to the churches, written perhaps 20 years later, had to address difficulties which had arisen. But here in Jerusalem, at that moment, nothing was spoiling the Christians’ euphoria.

But is this actually true? Some Bible scholars suggest that what we have here is an idealized picture of the Church’s life, that these verses describe a state of affairs that never actually existed. They believe that Luke (who wasn’t present in Jerusalem as he didn’t come from there and wasn’t converted until years later) was keen to give us a picture of what the Church ought to look like if it were perfect – an aspiration to aim for. However other writers – and I’d prefer to go along with them – reject this approach, which they call ‘cynical’ and ‘pessimistic’. They say that, yes, the members of this community were ordinary folk with all the usual human weaknesses and differences – but that such a negative view fails to take seriously the way in which people can be changed by the power of God’s Spirit and the Gospel of Jesus Christ.

At this point I want to introduce you to a Greek word which the New Testament uses quite often when talking about the Church: koinonia. It’s often translated as “fellowship” but that doesn’t really do it justice as that can mean little more than casual acquaintance and superficial chatting. Koinonia goes much deeper and refers a community which involves close-knit sharing and participation amongst its people. One writer defines it as “a divinely intimate and holy unity among believers – and between believers and the Lord – which involves everything from oneness in the Holy Spirit, community life, sharing contributions from money to food gifts, and the Communion partaken in the body and blood of Christ Jesus”; someone else calls it “a shared experience of salvation”. Others go further still, calling koinonia “the source and end of the Church’s existence” and saying that it’s a foretaste of the perfect and complete fellowship we believe we will enjoy with each other and Jesus following his return. Clearly this is far more than a brief chat at the church door after Sunday services!

This strength of community, this depth of fellowship, sounds scary. That’s partly because our society encourages autonomy or a “me-first” mentality, from “doing what makes you happy” to constantly taking selfies and posting them online. We make decisions or cast our votes in ways which place our own interests above those of wider society. We like to come home and shut the front door on our neighbours and the outside world (how dare they interfere in our lives?) – so different to the shared joys and hardships of old-time life in rural villages or the industrial Valleys (which could of course be claustrophobic). And we may think of faith as something that’s intensely personal, between “me” and “my Lord”, with churchgoing (so called) as an optional extra.

I also think we’re scared of deep fellowship because it makes demands of us. It may force us to “love our Christian neighbour” in practical, time-consuming and inconvenient ways. It may force us to push aside the protective emotional armour we’ve built around us, the façade we like to show to the world, and engage with other people in ways which may expose our ignorance or be emotionally draining. This may sound cult-like, but it shouldn’t. We must remember that Christianity, while it gloriously values individual people, consistently urges us to place God first, others second, and ourselves (possibly) last. For, as Jesus said, the two greatest commandments are to love the Lord with every ounce of our being, and to love our neighbour as much as ourselves.

So is this church – our church – a community in the New Testament sense? Well, I think most of us would say that we’re a friendly church, that we are pretty good as welcoming folk, that we rub along fairly well together most of the time – but is that enough? Are we living and practising this koinonia as we ought to be? We might think we are – but I’m not altogether sure. Someone pointed out to me just this week that, when most of us come into church, we tend to greet and chat with our friends but aren’t so good at mingling with visitors. And I’ve noticed that, when we have coffee in the Hall, we tend to split into three distinct groups – long-standing attenders (who are, dare I say, mostly white), newcomers (who are mostly black) and young people.

Now I can understand why this happens, but it isn’t right. It’s been thrilling to have had a number of new friends joining us over the last year or two: many churches would give their eyeteeth (whatever those are!) for the privilege. But we want them to stay, we want them to feel valued, we want them to feel included, and we’d love them to take on various responsibilities. But this will only happen if those who have been here for years make the effort to reach out to them. That’s why I asked you all to get up and greet someone you don’t know very well at the start of this service. But that’s only a first and tiny, step: all of us (and I include myself in this) need to go much further in crossing our boundaries of discomfort and even fear. After all, that’s something that Jesus did: as so often, he shows us the way.

I’d like to finish by taking you back to that first group of Christians in Jerusalem. You might well say, “It was easy for them to get on so well as they were all Jews, they all came from much the same background. But the world today – at least in Cardiff! – is a far more complicated place”. Well, there is some truth in that, and the early Church did have to confront quite a few difficulties when Gentile people such as Greeks, Romans and Africans wanted to join it. However we mustn’t forget that, on the day of Pentecost, Jews from all around the known world had gathered in Jerusalem, including among others Parthians, Medes and Egyptians, Cappadocians, Phrygians and Pamphylians – names which every Bible reader struggles with on Whitsunday! In other words people from a huge range of backgrounds were present and it’s surely likely that some of them were not only converted to Christianity that day but stayed on in the city and joined the Church.

We mustn’t make the mistake of thinking that people who look the same, or who come from the same background, are identical. To paraphrase “Monty Python’s Life of Brian”, we’re all individuals. Rodney Sadler, a distinguished African-American theologian at Union Presbyterian Seminary, has written this about Black Christians in his country: “We may share a common hue, but that doesn’t mean we are all alike. We are different – different because of class, career, calling, and concepts. The “black community” is more of a fictive concept than a discernible reality. Even the languages we speak are different. Anyone raised in the era of Marvin Gaye, the Jackson 5, and the Supremes can attest to this if they have ever tried to communicate with those from the Jay-Z, P-Diddy, and Destiny’s Child’s era; we do not all speak the same language! The 2008 primary elections divided the African-American vote between Barack Obama and Hillary Clinton, and the 2004 presidential election divided it over abortion and gay marriage, showing us that our political views are no longer routinely similar. We are different. No longer is it even certain that our church members who commute from the suburbs to our inner-city churches feel affinity for those un-churched brothers and sisters they pass-by along the way”.

He concludes: “As in our churches, the crowd in Jerusalem, despite their differences, were assembled only because of their common faith. Those mixed Jews were remixed and united by the Spirit into a new way of being community in the world. That motley crew was united by the power of the Holy Spirit who bridges the differences between them and makes of many languages, voices and backgrounds, one community in Christ”. That is true for any group of Christians, whether they look and sound much the same and seem to have lives which are similar, or whether they are a hugely varied bunch of people. Jesus wants us to be a real community – one which looks so attractive from outside that people are drawn to joining it. But even though the Holy Spirit helps us to build that community, it doesn’t come into existence by itself. We have to want it to exist, and we have to put in the effort to create and sustain it.