The decision by Gibson on no subtitles is brilliant. It forces Gibson to present the universal humanity and inhumanity detailed in the Passion. Every person who sees it, despite what language they speak or their religious background, will know what is happening because it will be so human. What a tremendous task!
One of my favorite movies is "Salaam Bombay," which was shot in Hindi. By mistake, I watched it recently with the subtitles off. It was almost like watching a different movie ... even more beautiful than I had found it before. The images truly spoke for themselves. I think that's the effect Gibson is trying to achieve, and I hope he does.
It is hard to say without having seen what he's lensed already. It takes remarkable directorial talent to tell a story entirely by visuals.
Devout Christians will of course know the story by heart and even the lines. The uninitiated, however, may not. Gibson may want to consider subtitling just key lines of dialogue. But he's the filmmaker. I certainly am not. If he has a vision and he is passionate about it, it's his decision to make, and we can only hope a distributor agrees to take the risk without conditions.
Perhaps if nothing else Gibson ought to consider a subtitle option when the DVD is released.