We know that it’s wrong to physically harm people. But is it ever right to hurt people’s feelings for the sake of God’s truth? For instance, Jesus called the Pharisees a “brood of vipers.”
The phrase “for the sake of God’s truth” makes a big difference. In the passage you cite (Matt. 23:29-36), Jesus isn’t so much insulting the Pharisees as using a vivid metaphor as part of a prophetic critique of their mindset and their actions. In Scripture, snakes are sometimes associated with deception and temptation, and the viper, of course, is a poisonous snake. So Jesus is criticizing some of the leaders of his day for a dangerous deception that was leading Israel away from God’s call.
That leads to the question of whether Jesus’s approach ought to be a model for us. At least two considerations are important: First, what is our intention when we address those we think are wrong? It should not be to insult or demean, but to testify to the truth. There might be times when strong, potentially hurtful rhetoric is appropriate. But any hurting of feelings should be incidental to arguments toward justice, mercy, and truth rather than intended as insult.
Second, there is the issue of strategy and effectiveness. If our goal is to persuade those we think are wrong, then we should think twice before using rhetoric that labels them in strongly pejorative ways. We all know how easy it is to adopt a defensive, counterattack mentality when we feel attacked. Of course, sometimes we experience even legitimate critique as a personal attack. In short, there is no guarantee that any of the approaches we take in our arguments on behalf of God’s truth will be received in the way we intend.
In all of this, while we Christians are indeed called to testify to the truth, we also should bear in mind the fact that we sometimes get things wrong. And we should remember Jesus’ warnings about self-righteousness and judgmentalism. Awareness of the planks in our own eyes will go a long way toward helping our dialogue partners to consider the specks in theirs.