The passage speaks about catholicity, recognising the universal church and rejecting factionalism. We do this in two ways: firstly by treating churches and Christians that differ from us humbly and with generosity of Spirit (vs 38-42); and secondly, by seeking holiness in our own lives (vs 43-49) Thus we have salt in ourselves and peace with each other. (vs 50) The passage itself gives two reasons why we should seek catholicity: the reward for helping others who serve Christ is great (vs 41), and opposing the work of God is a serious sin.(vs 42) The benefits of this approach are shown helpfully in the way the church at Ephesus dealt with Apollos (Acts 18:24-28), and Paul (Gal 1:11-2:9). There are three other reasons why we should show humility and generosity to other churches:
God’s Vision of the Church is Bigger Than Yours. We tend to imagine that there is one way of doing church – ours! But God’s vision of the the Church is a heavenly city, full of people of every tribe, tongue and nation, and all the treasures of the Earth. (Rev. 21) This means that we should expect to find a unity that is rich in variety. From this, and also the Church as a body (1 Cor 12), we see that if every church were the same the body of Christ would not function. Knowing this give us a rich resource for us and our churches.
God is More Gracious than You. We tend to think that God would only work through the best theology, practices or people, but he uses them even when they are imperfect because his Church is still being perfected: people are saved by inadequate Gospel messages and flawed methods, churches are successfully shepherded by weak and broken pastors, people are discipled despite wonky theology, and God is glorified in his people despite their imperfection. The Church is a bride being prepared and made perfect, we must love her even now as Jesus does. This is good news for us, knowing that God can use us for his glory and to share his Gospel despite our inadequacies.
The Superabundant Goodness of God. The Lord makes the sun to shine on the righteous and unrighteous alike, it is his very nature to do good wherever and to whoever he can. He used an ass to speak to Balaam, and Balaam – a pagan diviner – to speak to the Israelites. And so God uses ministers, churches and theologies that aren’t just different, or imperfect, but even those which are actually wrong to minister his grace to people. Realising this actually enables us to exercise discernment. Just because God continues to do good to someone or through someone it doesn’t meant that he approves or them or their ministry, nor that their theology or practice is a legitimate expression of the Church’s catholicity.
a) God does work in extraordinary ways, outside of the Church, but that doesn’t mean we shouldn’t seek to establish norms, nor that we should emphasise the extraordinary over the ordinary (e.g. revival chasing).
b) We cannot have fellowship with those who are persist in explicit or implicit schism. (Tit. 3:10-11).
c) In seeking unity we should be less inclined to judge the problems in other church and focus more on our own holiness. As we seek to be holy – salted with fire (vs 49) – we understand all of God’s work more and recognise more clearly our brothers and sisters in all of their beautiful diversity.