Mysticism in the proper sense is an intense realisation of God within the self, and the self embraced within God in vivid nearness.
It is a phenomenon known in a number of religions, and in those religions very similar language is used in describing the experience. There is a deep darkness, the darkness of not knowing, and there is light, with flashes in which the self knows the unknowable to be terribly near, and knows itself as never before.
Now, through the centuries Christian teaching has emphasised that the significant thing is not just the mystic experience in itself, but its place and context within the whole life of a Christian. The experience is given by God sometimes to one who seeks God in a life of humility and charity, turned towards righteousness as well as the beauty of God. And the effect of the experience of mystic union, sometimes described as 'passive contemplation', is not to cause the person to long to have the experience again, but to long to serve God and to do his will.
Those who have had mystic experiences will not want to tell everyone about it: they will have a longing to serve God in daily life, for in his will is our peace.
Michael Ramsey, Canterbury Pilgrim