It may be tempting to think the church can win converts if it has the right strapline or soundbite, but the most effective way remains telling people about Jesus Christ, says the former principal of the London School of Theology.

Speaking at this year’s Keswick Convention in Cumbria, Derek Tidball said the church often wanted to “keep the Gospel back” and “hook people by other messages first”.

“Judging by the way the church in England behaves, we don't have confidence in the Gospel,” he said.

Tidball argued that the simple teaching of the Gospel and “the unpacking of the unsearchable riches of Christ” were still the most persuasive ways to bring people to a genuine point of conversion and discipleship.

He said: “I have been, over the years, to many conferences on evangelism that have reduced evangelism to marketing and suggested that, if we only get the right language, the right strapline, the right soundbite, the right techniques – if only we can tap enough into the culture, then of course it will be obvious, everyone will see the truth of the Gospel and come to believe. But it doesn't work like that. We are not selling cars or soap powder. We are engaged in a spiritual battle.”

He called upon Christians to reconsider their priorities and live a life of serving and showing mercy to others.

“I sometimes think that the Church is not particularly known for its mercy. It is known for its judgmentalism; its tut-tutting; its objecting to this, that or the other. But the God in whom we believe is one who is merciful,” he said.

The Keswick Convention is an evangelical conference that takes place each year over several weeks of the summer in the Lake District. It is being attended by some 12,000 Christians.

Earlier in the conference, the international director of Langham Preaching Jonathan Lamb warned that Western Europe had become a region of “empty nominalism” and “confused spirituality” in which even millions of people calling themselves Christian were only Christian by culture or tradition.

He also urged Christians to be on their guard against any models of Christian life that tried to avoid weakness and promoted the idea that those who face hardships or trials are outside of God’s blessings.