Who actually speaks on behalf of Christianity? What is the official beliefs? Christian orthodoxy persecuted and destroyed heretics through history, does that make it the real version of Christianity?

Of course Warren reflects the view of many American evangelical Christians, and as far as I'm aware, there is no such thing as an interfaith view or interfaith religion, with the exception of perhaps that strange religion known as Baha'i. The entire interfaith dialogue is an attempt for various differing viewpoints to form a civilized dialogue between them, and not to assimilate all those different views into one single monotheistic interfaith viewpoint, which is absurd.

So the interfaith thing is a myth. It's propaganda by naive people trying to resolve what is unresolvable, trying to make peace with views that can never be at peace.