The evangelical leader hopes the forum will provide Americans with a more in-depth look at the candidates, especially their values, visions and virtues.
"Many evangelicals think neither of these guys are evangelical," Warren said.
Evangelical white Protestants account for about two in 10 Americans, with lopsided voting patterns that give them clout, such as in 1994, when they helped the Republican Party gain control of Congress for the first time in 40 years.
In the most recent ABC News/Washington Post poll evangelical white Protestants were 28 percentage points more likely than other Americans to identify themselves as Republicans (47 percent vs. 19 percent) and 27 points more likely to be conservatives (57 percent vs. 30 percent).
Seven in 10 call "strength and experience" in the next president more important than "a new direction and new ideas," again among the highest of any group. Sixty-two percent say McCain shares their values; among all other Americans just 44 percent say the same. Just 36 percent say Obama shares their values. Among other Americans, it's 61 percent.
"I think many evangelicals are saying, well, I like John McCain and I like Barack Obama, but I'm not really sure I like this about him," said Warren.
Both candidates identify themselves as devout Christians who have struggled to convey their faith in a country where religion is as important as it is controversial.
McCain, an Arizona Republican, has not always seemed comfortable talking about his faith and in the past has said that while he was unashamed and unembarrassed by his "deep" faith, he had no intention of imposing his views on others.
In the past, the presumptive nominee has had a rocky relationship with some conservative Christian leaders, and he is not doing as well in the polls with evangelicals as he would like.
Obama seems more comfortable talking about religion, claiming his faith encompassed a commitment to Christ and community. But rumors that the Illinois Democrat is a Muslim, his controversial former pastor and his liberal views on abortion and other issues complicate his efforts to win over evangelical voters.
"I've had a personal relationship with Christ now for almost 50 years. He's my best friend. I talk to him all the time," Warren said.
When asked whether outreach efforts by the presidential hopefuls, specifically those of Obama's campaign, would help to win over evangelical voters, Warren said, "I'm a pastor, not a prophet."