Jesus said: "Whoever is not against us is for us." (Mark 9:40)

There is a cartoon you can find dispersed across the internet which always makes me smile.

It features a teacher standing in front of a chart showing ever sub-dividing groups of Christians from the earliest days until now. The teacher is saying to their class: "So this is where our movement came along and finally got the Bible right," to which one of the students responds: "Jesus is so lucky to have us!"

In a way that sums up the truth of what Jesus is saying in Mark 9:38-43. One of his friends has told him: "We saw someone casting out demons in your name, and we tried to stop him, because he was not following us."

But Jesus replies: "Do not stop him; for no-one who does a deed of power in my name will be able soon afterwards to speak evil of me. Whoever is not against us is for us."

Writing on this passage, Jeremy McQuoid, a speaker from the conservative evangelical Keswick Convention, observes: "Evangelicals should not become a private clique that believes that God cannot be at work outside their elite group. That is arrogant and unworthy of the cross."

He is quite right. Elitism and fragmentation seem to have plagued evangelicals – and indeed many other Christian streams of thought – from the earliest days (as these verses in Mark demonstrate!).

Why is this? One reason is simple: if you believe you have truth, it is easy to slip into thinking that everyone else must always be in error. As Professor J.I. Packer commented as long ago as 1978, the conviction of evangelicals "that God has given them a deposit of truth to guard, and their resolve not to let it slip away or be relativized or distorted, surely merits praise." Absolutely!

But he goes on to say that the accompanying "assumption (hidden, yet potent) that evangelicals really have all the truth, and that God would never show Catholics or non-conservative Protestants anything that he had not first shown to evangelicals, making it needless for evangelicals ever to learn from those quarters," is "really absurd."

A second reason is perhaps a more human one. John Stott wrote in 1995: "We fragment over what we regard as issues of principle – but often the real reason is personal, isn't it? When we're afraid, we withdraw into our own fellowships and ghettos where we feel secure with like-minded people." It was a shrewd observation.

As long ago as the 17th century the conservative writer Richard Baxter, in The Reformed Pastor, lamented those "who tear their brethren as heretics, before they understand them."

We need to be slightly careful, however. In the first place, Jesus says elsewhere (Matthew 12:30, Luke 11:23) that "whoever is not for me is against me." Following Jesus has always been a black and white matter! (Although it's possible Jesus is speaking in a somewhat different context in those verses).

Secondly, Jesus is not saying that truth and error are negotiable. Elsewhere, he warns against "false prophets" (Matthew 7:15). So we always need to be very discerning. And he is scathing both about doctrinal error (Mark 12:24) and hypocritical behaviour (Matthew 23:1-36) among religious leaders. So we cannot always "compassionately disagree" about anything and everything – especially matters that affect people's salvation (as Jesus is about to go on to say, in Mark 9:42).

But if Jesus is for truth he is against tribalism. As the puritan Thomas Goodwin wrote: "I have found gospel holiness where you would little think it to be, and so likewise truth. And I have learned this principle, which I hope I shall never lay down till I am swallowed up of immortality, and that is, to acknowledge every truth and every goodness wherever I find it." Amen!

The Rough Guide to Discipleship is a fortnightly devotional series. David Baker is a former daily newspaper journalist now working as an Anglican minister in Sussex, England.