Perennialism is the idea that all religions participate in the ultimate truths. This contrasts with perhaps the Buddhist who feels that it's the Buddha's experience that matters, and that there isn't some universal kind of truth it participates in.
As I delve into the book, I'm don't think I want to be a traditionalist, and I think it's OK to blend modern ideas with Buddhism. I don't necessarily think science has the best truths, or that Buddhism is more scientific than other religions. But I do believe in pluralism and multiculturalism instead of separatism or the importance of erasing differences.
Perennialism really helps in the exclusive truth wars. I've written in the past about friends who think religion necessarily leads to people feeling closer to the truth and that leads people towards conflict. A modern mindset is inclined to think that truth is more of a function of a system, learning the lessons from Godel's Incompleteness theorem. That you can't prove the truth of a system without referring to the system, and that just makes it all a kind of game. There is no super system that explains it all. That points to a kind of pragmatism, just noticing what each system does, and resisting reduction or assimilation into other systems. Perennialism functions to promote harmony, and not discord, and whether it's true or not. Nobody needs to be colonized or converted, we're all on the same journey. The only danger could be to ignore differences and history.
The modern world has allowed for more open source Buddhism, that teachings are available. We don't need a system that concentrates the information into the powerful hands of a central authority. We don't need to have an allegiance to one kind of Buddhism, we can love the variety and appreciate the spirit of certain teachers, even if we don't believe in the system they created. We own our own spirituality and have the responsibility of finding out what works for us. Now you can lose some depth by not committing to one tradition and there will be people that will tell you you should only shop exclusively in their bookshop, but most people see through that. They see it as branding, a sales pitch.
The lack of a cohesive system and community can be an outgrowth of open source Buddhism. The real living traditions that actually exist are more traditional.
I like to be around other people of different traditions, and I get the feeling I sometimes get when I'm with sangha. Even outside of Buddhism.
So it think the trick is to have a community, with a tradition, but that also allows for independent thinking. And being aware what is Buddhism and what is not Buddhism. What does our modern culture add in to adapt Buddhism to our times, and what do we gain and lose by doing that? I don't like sectarianism, but loosey goosey doesn't always work either. Buddhism without beliefs is wonderful, but Stephen Batchelor has not founded a sangha. He visits other sanghas and organizations. People come to hear him teach, but he does not present a system for spiritual development.
I think perennialism, which would be hard to prove, does function in a harmonizing multicultural way.