|Existence blossoms out of non-existence, and no-one knows why.|
You cannot conclusively prove that your memories, experiences and senses do not deceive you (and in fact your experience will be that they do at times deceive you). Therefore the only thing you can be truly certain of, to your core, is that something that you experience as self exists. Sometimes expressed as "there is thought, therefore there is a thinker". The only certainty is I Am.
Therefore, certainty cannot be the basis of our knowledge of other, and the ways of other. We just can't have it. If you think you are certain, then you are provably wrong. Thinking you are right is not a way of thinking that can lead you closer to truth. We must accept and embrace uncertainty.
This brings fear, for if anything might be, and anything could be true then the worst might happen and we know the worst is so very bad because we can imagine it.
But if the worst might happen, then the best might happen too. And the best is so very, very good. So uncertainty is something to love, for it makes the best possible. The future is, in large measure, what we collectively make it.
Let's expend our imagination on the best, and deal with the worst as much as we are required and no more.
To carry on from Descartes, and which is the assertion of Buddhism, what if when I was utterly still the experience of self disappeared completely. And then I could know nothing at all. If my experience of self is only found in motion and activity or thought. What then?
I have absolutely no idea.
The response of Christianity is that beyond ego death is resurrection life. Utterly pure life. The way to attain it is the path of laying aside self, a way of life of putting others first. The laying down of one's life. This is the way of love, the way of the Risen Lord. A beautiful metaphor. It also happens to be undefeatable.
We can participate in the mind of Christ. It is no longer I who lives, but Christ who lives within me. And I am made completely myself.
In this living metaphor, the Christ is the personhood of perfection itself.
To live in the perfection of it would be motion and activity and thought, but without self.
In the infinite thought-space of every possible idea, there is one that I fear above all. For in it is annihilation, the little death.
It's a tiny thought, perhaps the smallest possible, and only one in the swirling void without end of infinite mind. But it's special.
Alas and alack for me this alluring gem of the mind has a special beauty, an ineluctable draw.
This thought is the one truly infinite thought the human mind can contain. The thought of this thought has me captured.
This idea is nothing. Not even emptiness, the lack of a void. Nothing at all.
Such a simple thought, a tiny idea. But who can bear it?
Alone without even yourself.
Spirituality in particular can be to us a rich and broad set of metaphors, a higher abstraction for thinking about, discussing and participating in the flow and patterns of life. An abstraction we find in the mythos of cultures and fiction and woven into reality around us. The collective dreamings and imaginings of all of humanity past and present.
Knowing our way back to ground we are safe to fly. Our conceptions need not be "ultimate truth" to us, but useful abstractions and ways of thinking that we can put weight on but we understand that they are only ideas and ways of thinking that inform and shape our experience. What is truly important is what is here and now right in front of us and the duty and responsibility of caring for those we love -- and that is what we hold to be most real. We are always willing to be wrong and to reevaluate our ideas in the light of new experience.
One precious thing my psychosis taught me is that the way I see the world is fallible, that my memory is fallible and my understanding is fallible. No matter how heartfelt and genuine my belief is, no matter how strong my faith, I can be wrong.
This incidentally also gives us a safe way to trust people. "Trust but verify". We can trust by default, accept by default and love by default. But we keep our eyes open, it isn't a blind faith.