The distinction between 'numinous' and 'mystical' experiences comes from the academic field of phenomenology in comparative religion. Numinous refers to experiences of a divinity apprehended as distinctly 'other' than oneself, usually characterised by trembling, awe and potentially fear. The term was coined by Rudolf Otto. See http://en.wikipedia.org/wiki/Numinous
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Mystical in this context refers to a sense of the divine or holy experienced as non-distinct or in unity with oneself. Notions of the universe as 'all one' are characteristic.
These two forms of religious experience are regarded as opposing trends within the academic field by virtue of the importance placed on 'otherness' versus 'oneness'. My feeling on the other hand, is that they form a continuity of experience, whereby the numinous is preliminary to the mystical. This type of progression is seen in the life of the Hindu saint Sri Ramakrishna, for example, of whom it is said he was forced to violently attack his well established numinous vision of Kali in order to experience the higher state of nirvikalpa samadhi. Also in the medieval mystical theology of Christianity which uses the progression of via purgativa, via illuminitiva
and via unitiva
where the numinous experience belongs to the path of illumination while the mystical experience is obviously unitive. I was hoping to find a reference to a source which discusses different forms of visualisation in Buddhist deity yoga as a progression of this sort to further support my contention.