Warren is becoming pretty influential in presidential politics:

The Rev. Rick Warren has persuaded the candidates to attend a forum at his Saddleback Church, in Lake Forest, Calif., on Aug. 16. In an interview, Mr. Warren said over the weekend that the presidential candidates would appear together for a moment but that he would interview them in succession at his megachurch.
[…]
Mr. Warren, the author of the best-selling book “The Purpose-Driven Life,” said he had called each man personally to invite him to his event, which will focus on how they make decisions and on some of Mr. Warren’s main areas of focus, like AIDS, poverty and the environment.
[…]
Mr. Warren said he would devise his questions with input from the Muslim, Jewish and Christian leaders associated with the group.

The McCain campaign should have someone coach him before the forum on how to look comfortable talking about his faith or this could go pretty badly for him.