Hi Garro, and welcome.
I wonder is there other people who have taken on not knowing as the key to their belief system?
I think every system encompasses this aspect. In Christianity it is the apophatic tradition, the way of not knowing.
Is it possible to not know and still be considered a spiritual person?
Consider the question.
For a start I would say everyone is a spiritual person, the person is a unity of body, soul and spirit, therefore what do we mean by 'a spiritual person'? It can only be the recognition of a given quality, which we term 'spirit', in which case, if one does not know, one wouldn't be able to recognise it. And if one does, then one needs first to know what it is one recognises.
So I would say no, it's not possible to not know and know at the same time.
To know the distinction between knower and the known, on the other hand, is something else altogether. I tend to think the 'not knowing' is then not the deficiency of knowledge (the known), but the recognition of the unknowable-ness of the knower ... to know something truly as it is, one must be able to view it, as it were, from all sides. To contain it.
Man can contain an infinite number of things in mind, but even the sum of all he knows will never equal the startling and primordial fact that he is a creature who knows ... and moreover a creature who knows he knows ...
To know in itself is greater than the sum of all possible knowledge, to know is infinite whereas knowledge renders things finite.
The apophatic way of knowing then is not access to a body of knowledge, esoteric or otherwise, rather it is a knowing according to being, rather than a knowing according to a theory of knowledge. This is a 'dark knowing' or 'divine ignorance', but it is full and rich. It is properly called Faith.
Many like to assume 'faith' is a deficiency of knowledge — this is incorrect (ignorance is). Faith transcends knowledge because it speaks in essence and not in form, of being and not knowing.
In Catholicism we have the Gospel of John and the Gospel of Paul (in his letters) which both speak of not knowing, and a way of being that is a new creation in Christ.
Then, in the world's view of the mystical tradition, we have some pretty heavy-hitters — Clement and Origen, Dionysius the pseudoAreopagite, St Gregory of Nyssa, St Maximus the Confessor, the author of The Cloud of Unknowing, St Thomas Aquinas and St Bonaventure, Nicholas of Cusa, Meister Eckhart and my favourite, Johannes Scottus Eriugena, who was about a 1,000 years ahead of his time.
People who are willing to except the mystery for what it is. Does such a group of people exist?
In every tradition, yes ... but finding them ...
BTW, it depends upon how you define 'mystery'. In the common sense today, it means an unknown, a who-dunnit, or perhaps what-isit, a problem without solution, in which case it seems you mean mystery in a scientific sense — how the world is. And certainly, thanks to microscopes, telescopes and HD-TV, there's plenty enough in nature to astound and beguile the senses.
If however you mean 'mystery' in the traditional, spiritual sense, then the question is not how the world is, but why the world is.
Mystery in the Christian sense, for example, is not 'the unknown' but rather 'the made known' — for there to be a mystery means something must first be revealed — the veil, the archetypal symbol of the mystery, reveals and conceals simultaneously.
God bless,
Thomas