They came from different worlds: Rick Warren was the conservative white pastor of a 20,000-strong evangelical church in Orange County; Illinois Sen. Barack Obama was a liberal black politician, and a rising star in the Democratic Party.
After meeting in Washington, D.C., in January, they started chatting regularly on the phone. When Obama was writing his best-selling book, "The Audacity of Hope," he asked Warren, himself a best-selling author, to review the chapter on faith.
As Warren planned a second international conference on AIDS at his Saddleback Church in Lake Forest, he asked Obama to address the group during a session Friday titled "We Must Work Together."
Some evangelicals had already criticized Warren for his different approach toward AIDS, which included working with gays. But the speech by the pro-choice potential presidential contender has drawn renewed vitriol from conservative Christian radio hosts and pundits, as well as some evangelical preachers.
"Why would Warren marry the moral equivalency of his pulpit -- a sacred piece of honor in evangelical traditions -- to the inhumane, sick and sinister evil that Obama has worked for as a legislator?" wrote radio host and blogger Kevin McCullough.
Saddleback Church responded to the criticism with a statement Wednesday defending Obama's appearance at the conference, but also noted Warren's disapproval of some of his political beliefs.
"Let it be made very clear that Pastor Warren and Saddleback Church completely disagree with Obama's views on abortion and other positions he has taken, and have told him so in a public meeting on Capitol Hill," the statement said.
"Our goal has been to put people together who normally won't even speak to each other. We do not expect all participants in the Summit discussion to agree with all of our Evangelical beliefs. However, the HIV/AIDS pandemic cannot be fought by Evangelicals alone."
But the evangelicals' foray into AIDS work is a relatively recent development. According to religious scholars, they were among the loudest voices insisting AIDS was God's punishment for gays' behavior after the disease emerged in 1981. They remain one of the religious groups slowest to respond to the pandemic because of the disease's links to homosexuality and promiscuity, prohibited by their interpretation of the Bible.
"This is a touchy subject for evangelicals," said John C. Green, a professor of religion and politics at the University of Akron and co-author of "Religion and the Culture Wars: Dispatches From the Front." The conference "really is a departure, [but] you'll probably find a lot of the ambivalence really hasn't gone away."
Warren's wife, Kay, agrees that AIDS has been a difficult subject to broach.
"Evangelicals have been really afraid," she said. "They don't want to talk about condoms. They don't want to talk about HIV because that means having to talk about sex. We want to break that kind of silence."
Kay Warren became aware of the vastness of the problem in 2002, after she read a news magazine article about the 12 million orphans the disease had left in Africa.
This discovery spurred her to visit Mozambique, where she met an emaciated woman lying beneath a tree who was dying of AIDS. Warren grew angry that everyone -- the woman's family, her church, her government -- had abandoned her. Then, she realized she, her husband and their church in an affluent Orange County community were guilty of the same indifference.
"We had done nothing, we had done absolutely nothing," she said. "That hit me like a ton of bricks. Instead of being judgmental about what wasn't being done in other places, since we were doing nothing, we had to come back and repent. We have been so wrong. We haven't cared. We haven't said one word."
The fruit of that revelation is unfolding today at Saddleback Church, where hundreds of scientists, pastors and caregivers will meet at the AIDS conference.
"I have no doubt if Jesus were walking the Earth today, he would be hanging out with people with AIDS," said Rick Warren, author of "The Purpose-Driven Life," the Hawaiian shirt-wearing founder of Saddleback and among the nation's most influential evangelical preachers.
His wife was the catalyst who drove his concern over the disease.
"I realized the face of AIDS was more than just a white gay guy," he said. "It's all kinds, black, brown, women and children, much bigger than I realized. I had no idea what a big deal it was."
The Warrens believe that the vast network of houses of worship throughout the world, acting in concert with government, businesses and nonprofits, can provide medical treatment, nutrition and services to people with AIDS. While they do not condone some of the behavior that may lead to the spread of AIDS, the Warrens said it is their sacred duty to show mercy to anyone the disease afflicts.