The concept of the Holy Theatre comes from Peter Brook's The Empty Space. Brook divides his book into four essays, describing different types of theatre, including a holy theatre.
Except that he spends most of his essay on the Holy Theatre failing to find a definition for it. He describes experiences of Beckett, Artaud, Grotowski, but never defines what could make theatre holy.
For me, holy theatre is a place of open revelation. A space where I can enter into a relation with the divine through art. It is not a space I have visited often, but I have been there often enough to fully appreciate its value. Many times in Unitarian Universalist circles, we ask about where we have experienced the divine or something greater than ourselves. Most of the answers are about quiet reflection in nature.
The moment that I have felt most in touch with the divine was when I was at the back of the Auditorium Theatre watching the Alvin Ailey Dance Company perform Ailey's masterwork, Revelations. The audience was one with the dancers and we all were performing a sacred ritual together.
I've had similar experiences of audience/artist fusion at The House Theatre of Chicago. When the audience moves from a passive participant in the process, to an active (yet respectful) participant, that is holy for me. I want my theatre and my worship to require me to be fully present, fully a part of this current moment. That's what I seek in a holy theatre.