Let me see if I can help - seeing I was the first to create the word "Opethian". [at least I think I was].
When I came up with Opethian, it was my explanation of the unexplained reason why I was drawn in by the aura of Opeth. I knew immediately that the music was a gateway to something more, and when my vision became clear, I felt Opethian. Yes, their music is some of the greatest works I have heard. But hell, look at this messageboard. This is more than any other I have visited.
I don't think there is an individual Opethian moment other than the initial time you realized the big picture. Beyond that, you are already an Opethian who is encountering new experiences.