Good example, dualism and Christianity. I think this is a nice illustration of Wil's point, as Christianity has been engaging the topic ever since its beginning, in wildly non-linear, turbulent fashion. Arians, Monophysites, Unitarians, Paulicians, Cathars... All of these heretical movements were rejected by mainstream Christianity for being too linear, too uncompromising...
There have been very dark times in Christian history. I would like to believe we have moved beyond the times of inquisition, in at least it's outward forms. Nevertheless I think there is still a tendency, rooted deep in human nature, toward the thrill of a good witch hunt.
To see it without, I believe one must see it within first. Which one of us has not been at times the witch, the heretic, and at other times the inquisitor or the witch hammer? Or felt the imaginary pat on the back which seems to say, "Good job. But now, bring me another witch."?
If one does not see it within, it will always be "them", those out there somewhere, who are the problem. To see it within is to begin to lose the power to judge others.
I just finished a rather gritty book by Jon Ronson called "So You've Been Publicly Shamed". It deals with another aspect of what I believe is his favorite subject: the tendency toward group madness that is constantly lurking beneath the surface in us humans. (Here the inquisitors and witch hunters are tweeters and social media commentators). Bottom line, it leaves me with the uneasy thought that perhaps the human tendencies that drove the inquisitions of yesteryear are still alive and well in us today, just in a different form.
Regarding linear, as I see it in a religious context, means a progression. Fast, slow, full of starts and stops, at times seemingly going backwards to the point of despair, but ultimately not static. Moving not simply back to the place where we started, but to a better place. Circular, on the other hand, as I am able to grasp it, in it's most simple sense would be coming full circle back to the place we began, having made no progress.
If one believes we came from nowhere, we go back to nowhere. If one believes we came from a primordial place of simple bliss (Eden), we return to that place. However, these beliefs render our incarnation here meaningless, as there has been no progression. I believe this life has meaning and purpose, and is intended to lift man beyond his primordial origins. Not a return to his original state, but to a better one. (Like Paul's "much more").
Duality... I have my moments of non-duality, of non-conceptual existence, but I always come back with both feet firmly planted in duality (of the moral variety, good and evil). It's my world, it's where I live, I see it within and without, everywhere. I'm dual, but look forward to non-duality (which is really the non-conceptual, IMO). We say non-dual but really no words can describe it.
The trust is in the Giver, not in ourselves. The depth of love and the power to see it through are God's alone.