You are right in that Sheen stressed common moral/ethical precepts that could cross sectarian boundaries as "mere Christianity" or even "mere Theism". But it was difficult to ignore the fact that he was wearing the full archiepiscopal uniform, complete with cope, sash and pectoral cross, while doing so. You know - Funny, he doesn't look Lutheran.
Still, people found it very easy to ignore all that ~ in India the orthodox Jain Digambara achieve the same effect sans the clothing.
It's something to do with the quality of the teaching and the believability of the teacher.