How do we make our minds up about complex ethical issues? How can we decide between people we respect –Christians leaders, friends even – who offer conflicting advice about what the right decision is, what God's will is in a difficult or challenging situation?

An ever more common appeal is made to Jesus' saying in Matthew 7:16: "By their fruit you will know them."

At its worst, this saying is used to closed down any discussion of the issue at hand, and functions as a kind of shorthand for "If they are nice people, we ought to believe what they say." After all, if someone is kind, considerate and sounds pleasant – and that is how God wants us to be – how can they be misleading us over a decision about Christian life? It doesn't take much thought to see the problem with this.

There are plenty of verses in Scripture warning us of those who offer 'smooth words'; just check out Psalm 55:21, Isaiah 30:10, Daniel 11:32 or Romans 16:18. And in a passage which is well-known in some circles, Paul warns Timothy, as he passes on to him the baton of leadership, that "the time will come when people will not put up with sound doctrine. Instead, to suit their own desires, they will gather around them a great number of teachers to say what their itching ears want to hear" (2 Timothy 4:3). We only have to glance at the prophetic tradition to see that, very often, God called his servants to say some difficult things to his people that they really did not want to hear. Most of them would not have passed the 'fruit' test in the way it is commonly used today!

And yet that is only half the story. There are just as many times when the NT implores us to be gentle with our words and winsome with our speech, avoiding causing unnecessary offence even if we are expressing some hard truths. "Let your conversation be always full of grace, seasoned with salt," says Paul (Colossians 4:6). "Always be prepared to give an answer to everyone who asks you to give the reason for the hope that you have," says Peter (1 Peter 3:15) – but with a proviso: "do this with gentleness and respect." And it is equally clear that Christians often manifestly fail to do this. I wince inwardly when I remember the times that I acted as though it was more important to win the argument than win the person.

So how does the manner of our speech relate to the value of our argument? And what did Jesus mean by the saying about 'fruit'? There is one thing we can be sure he did not mean: that his teaching would be uncontroversial and easy to take. "I have not come to bring peace, but a sword" he claims (Matthew 10:34), to set people against each other, even within their own family. And, ironically, this saying comes in the context of Jesus sending his disciples out on a mission to proclaim the 'peace' of the kingdom of God (Matthew 10:13).

When we read Jesus talking about 'fruit', most of us read into this Paul's idea of the 'fruit of the Spirit' from Galatians 5, that is, personal qualities formed in us by the Spirit. But the language of 'fruit' in Matthew's Gospel has a different emphasis. When John the Baptist calls for 'fruit worthy of repentance' (Matthew 3:7), he isn't thinking about inner qualities so much as visible actions of obedience to God's commands (Luke gives some examples in the parallel passage Luke 3:7–14). Both John and Jesus link the lack of such 'fruit' to God's coming judgement – John is his teaching, and Jesus in the enacted parable of the withered fig tree (Matthew 21:18–19). In doing this, they are both linking the idea to the OT prophetic tradition, calling for the people to return to obedience to God. Deuteronomy offers two tests of the prophetic; the best known is the test in Deuteronomy 18:22, that if what the prophet has said "does not come to pass" then the word is not true. But Deuteronomy 13:1–6 gives a more important test; even if the prophet's words do come true, they are to be rejected if they lead the people away from God. Jeremiah 23 applies this to the prophets' own lives; if they don't demonstrate obedience to God themselves, don't listen to their teaching.

This doesn't completely solve our dilemma when presented with conflicting views on what is right. But it does help us when looking for 'fruit'; it is as much about conforming to the teaching of Scripture as it is about character. We cannot separate the two, and shouldn't use the latter to justify compromise on the former.

Rev Dr Ian Paul is associate minister at St Nic's, Nottingham, and Honorary Assistant Professor at the University of Nottingham. He blogs at psephizo.com and tweets as @Psephizo.