According to a story in the LA Times:

Yet even as the founder of Orange County’s Saddleback Church appeared to mollify those who have fought with him over gay marriage, he raised other eyebrows by invoking Jesus’ name and concluding with the Lord’s Prayer — both distinctly Christian practices on a day that has typically been characterized by more general expressions of “civil religion.”

“I don’t think he acquitted himself very well,” said Randall Balmer, a professor of American religious history at Columbia University who considers Warren a friend. “To lead the nation in saying the Lord’s Prayer, which is so particularly Christian, was a mistake.”

Here is a link to the Associated Press’s transcript of the sermon.