Traditional religion regards the bottom-line of knowledge as deriving from one or a combination of Tradition, Theology, Church authority and practices, or Scripture. These are external and 'objective' (i.e group consensus) sources.
By contrast, Mysticism is the conviction that the bottom-line source of knowing is subjective and experiential. Personal and subjective is regarded as potentially ultimately real; indeed the primary understanding of reality is subjective, personal, experiential.
In short: Mysticism is a kind of experience; and for 'the mystic', Mystical Experience is primary.
The primacy of Mystical Experience can be seen in its use both as validation and source. It may be used to check the validity of external sources of information, such as tradition, theology, church authority or scripture.
And Mystical Experience may also generate knowledge. For example, knowledge may arise in thought as ideas, concepts, information; or knowledge may arrive via perceptual communications - as with visions, or hearing voices - or combinations, as when reading the vision of a written text.
Thus Mystical knowledge is not defined by the nature of its content; but by how the knowledge is acquired. In other words, Mysticism is a theory of 'epistemology' (the philosophy of knowledge).
But an assertion of epistemology requires metaphysical foundation - because a theory of valid knowledge entails a metaphysical theory about the ultimate nature of reality - to explain why Mystical Experience is the most valid source of knowledge.
So, Mysticism is always (whether implicitly or explicitly) associated with an understanding of reality that explains how valid knowledge may arise by mystical means. For example, to explain why Mystical Experience is necessarily primary, or how subjective personal knowledge is also objective reality.
Mystical Christianity can therefore be understood as that Christianity for which knowledge of Jesus Christ - e.g. his identity, nature, teachings, mission, work, achievement, current role etc. - are primarily based-upon Mystical Experience; derived-from and/or validated-by subjective, personal experience.
Being a Mystical Christian first entails a conviction that Mystical experience is important, valuable - indeed essential. And having reached this conviction, will naturally set the stage for wanting Mystical Experience, for understanding (including checking) such experiences in oneself, and for developing Mystical capacities and practices.