The term mysticism is sometimes used in this day and age, or connote with someone who has a deep, spiritual experience.

noun: mysticism

belief that union with or absorption into the Deity or the absolute, or the spiritual apprehension of knowledge inaccessible to the intellect, may be attained through contemplation and self-surrender.
belief characterized by self-delusion or dreamy confusion of thought, especially when based on the assumption of occult qualities or mysterious agencies.

It is true that one who knows Christ is in a union with Him, as He is the absolute One to ultimately know. But it is also true that when one knows Him, that relationship, that union, is accessible to one’s intellect and mind. For as we know from scripture,

2 And be not conformed to this world: but be ye transformed by the renewing of your mind, that ye may prove what is that good, and acceptable, and perfect, will of God. Romans 12

And yet indeed, there are aspects of this union that one could consider as being mystical in essence. For sometimes one doesn't even know what or how to pray at times. Our spirit within us groans with words that are inexpressible:

26 In the same way the Spirit also helps our weakness; for we do not know how to pray as we should, but the Spirit Himself intercedes for us with groanings too deep for words;27 and He who searches the hearts knows what the mind of the Spirit is, because He intercedes for the saints according to the will of God. Romans 8

And when the Spirit himself begins to speak, either prophetically, or with words of wisdom of knowledge, these words don’t originate from our intellect. They spring forth from the very fountain of God's Spirit. They are perhaps mystical in the sense that they are unknown to those who don't know God. But to those who know Him, they are truth and life.

13 which things we also speak, not in words taught by human wisdom, but in those taught by the Spirit, combining spiritual thoughts with spiritual words. 1 Cor. 2

But what of those who are mystical but do not know Christ? What is the form of their mysticism and better yet, what is the origin of it? For the very word used to express the unknown, or the mysterious, is an expansive term that can oftentimes be either misused, or misunderstood.

St. John of the Cross would be one that we would describe as a mystic. And yet his Dark Night of the Soul, would certainly not be identified as one who didn’t know Christ. From his imprisonment as he sat month after month in a dark, damp cell, he came to know only God, for there was nothing else around him. He was alone in solitary confinement. John came to the light like many true mystics, and wrote beautiful pieces of poetry. And oftentimes, agony can lead to ecstasy, as seen in his prose. For his rigorous discipline, and abandonment, led him to a deep and abiding union with the cross and with God himself.

But to those who place experience and philosophy above the word of God, a dangerous door of mysticism opens up. When subjective, private spiritual experiences are sought without the aid of God's word for guidance or wisdom, then one can easily become lost and deceived. For they could conclude that their experience is valid, and that there is no need for any further proof. Indeed, then this door is then characterized by self-delusion and dreamy confusion of thought. For then the light has been obscured, and the angel of light has masqueraded as something that would seem to be attractive, but in the end, it leads only to more and more darkness.

There are many Christian mystics of the past, and yet even now, God is calling His own into a deeper union with Himself. It isn’t a union that is mysterious for those who experience it, but instead, it is the very relationship with God Himself. When Jesus said, 'I and the Father are one,' (John. 10:30) he proclaimed in himself the union of God and mankind, and this was offered to all who would follow Him. Those who are truly in union with Christ know that their life is hidden, and that it is no longer they that live, but Christ that lives within them.
I no longer live, but Christ lives in me! (Galations 2:20)

To know God intimately shows that mysticism is totally different from any form of legalistic Christianity. It means:

While we look to the scriptures for our belief and understanding of our faith, we also want to know Christ “personally.” This also means knowing Him, instead of just following after many of the rituals that we have grown to be accustomed to in the church. We also want to know Him, respecting the heritage we have received, but not to be restrained from developing a deep and intimate relationship with Him.

And while some seek out just experiences that are oftentimes mystical in essence, the Christian mystic develops a long-lasting experience of the eternal God. And this long-lasting experience ultimately leads to a transformation of one taking on the character of God. In this way then, it is the death of the “old man,” and the emergence of the “new man” in Christ.

Stephen Hanson