"The reconciliation is that in a pluralistic world…we [Christians] need to be on the same team because we share the same Saviour," Warren, of Purpose Driven fame, contended Sunday.
He was speaking with the dean of the Washington National Cathedral, Samuel T Lloyd III, who observed that evangelical churches are thriving and full of vitality, whilst most mainline denominations are confronting worrisome membership decline.
During the cathedral's weekly Sunday Forum: Critical Issues in the Light of Faith, Lloyd asked Warren how mainlines should tackle the problem.
"100 years ago the phrase 'social gospel' first came out," Warren responded. "Some people took that to mean only if we reform the social government and society and not personal faith in Christ Jesus – that is, if we make the world a better place – we don't need personal redemption."
That idea led to mainline churches going "one way" and evangelical churches another way, he said.
In general, mainline churches focused on social morality such as fighting poverty, racism and economic justice. Meanwhile, evangelical churches concentrated on personal morality such as personal salvation, fighting pornography, and upholding family values.
"Who's right? The fact is both are right," Warren emphasised. "Somehow we got divided like Jesus didn't care about society or members of society didn't need Jesus. I think we need both."
Warren called for "reconciliation" between mainline and evangelical churches and reminded the audience that Jesus taught his followers to love their God with all their heart, mind and soul, as well as to love their neighbours as themselves.
"You can't just love your neighbour; you [have] got to love God," Warren said. "And you can't just love God; you have to love your neighbours. And mainline protestant and evangelical – we need both wings."
Earlier in the programme, Warren spoke of his 20,000-member Saddleback Church, an evangelical church that has more than 400 ministries reaching out to different parts of society including prison ministry and programmes helping people infected with HIV/Aids.
The megachurch pastor also praised small church groups that meet in people's homes in strengthening Saddleback. The church has more than 3,600 small groups stretching 100 miles away from the main campus – or at least one in every city in southern California.
Warren said proudly that there are more people meeting in small groups each week – around 30,000 – than attending Sunday service.
"I could drop dead right now and the church would still be growing," Warren joked half-seriously.
The community-building guru added that the church is not built around one figure and that he spends only about half the year preaching at Saddleback and the rest travelling to speak and train pastors around the world.
Warren also highlighted that the evangelical church offers around 22 different worship styles across its campus on Sunday but listens to the same sermon.
"God likes variety," Warren said with a laugh.
Besides membership decline, many mainline churches also report decline in financial contribution.
Warren said his church does "zero" fundraising, but instead teaches biblical stewardship and generosity. Saddleback raised $7 million for Hurricane Katrina relief efforts and $1.6 million for the tsunami with one simple announcement made from the pulpit.
Personally, Warren and his wife Kay have led the church by example. The couple tithed the standard 10 per cent during their first year of marriage, but raised it one percent each year thereafter - 12 per cent during their second year or marriage, 13 per cent in their third year of marriage, and so forth.
But after the success of Warren's book The Purpose Driven Life, the couple started to practise 'reverse tithing' – giving 90 per cent to God's work and living on 10 per cent.
The Purpose Driven Life is the best-selling hardback in US history and one of the best selling books in the world.
Other points in Warren's talk on Sunday included his global P.E.A.C.E. Plan (Plant churches, Equip servant leaders, Assist the poor, Care for the sick and Educate the next generation) and his belief in the need for a second reformation - one where Christians change their behaviour to reflect what they claim to believe in.
Next week, Warren will attend the National Prayer Breakfast and hold a small meeting with DC-area pastors.