‘O generation of vipers’ – John the Baptist was a direct preacher. Few preachers have ever in their lives called anyone a viper.
The problem with the Pharisees is that they had no fear. They were not afraid of the consequences of their sin, because they did not see how great it was, and what they did see of it did not seem particularly serious to them. They did not fear death and coming into the hands of the Judge of all the earth. They had surrounded themselves with a cocoon of self-righteousness which shut out all qualms of conscience. There are modern approaches like the Alpha Course or like Cru (Campus Crusade for Christ), which are not frightened of death. Conversion is all about a human decision. You cannot be converted unless there is a fear of the Lord, a fear of the consequences of what we have done, a fear of the power of sin within us from which we need to be delivered. We don’t frighten people into the kingdom, but those who seek God need to fear what will happen if they do not come to him.