What is Christian Mysticism? Is it dangerous?
Christian mysticism refers to the development of mystical practices and theory within Christianity, but this is also sometimes referred to as mystical theology. The early church had a history of seeking the mysterious like visions, dreams, and times of deep meditation and prayer, but this can be problematic because if they are seeking a spiritual experience, they just might find it because there is another spirit out there, but he is an evil one. Satan can disguise his own ministers as angels of light (2 Cor 11:14), so it should not surprise us that Satan will counterfeit the truth by mixing a bit of error in with it, but this renders it a half-truth which is no truth at all. For those who seek a mysterious encounter with God, I would suggest you will encounter God in His Word, the Bible. There is no need to seek after the mystical when the clear voice of God is heard in Scripture. Why are we so fascinated by the mystical when we have the reality found in Jesus Christ? 
Gnosticism and Mysticism
If there is a similarity between Gnosticism and Mysticism, it would be that there is secret knowledge or a secret experience to be sought out. They believe that this kind of knowledge cannot be discovered by the mass majority. Gnosticism comes from the ancient Greek word “gnostikos” which means “having knowledge,” so Gnosticism is based upon knowledge, and they teach that material or physical matter is evil because the world was created imperfectly. To the Gnostics, God is unknowable, but even worse, Gnostics, just like those involved in mysticism, don’t even deal with sin because they believe salvation is achieved by getting in touch with the secret knowledge that most people do not have. What this does is create a prideful heart because they feel superior to others who are less knowledgeable about God. Mysticism, like Gnosticism, is highly speculative and subjective, while God’s Word is objective truth and is always true. Mystics may interpret dreams or visions as a message from God, but dreams and visions can come from another source too, and it’s not God.
Christians can easily get caught up in the mystic movement. At least they can have their appetites whet by it and start to seek after signs and visions, even though Jesus said an evil generation seeks after signs (Matt 12:39, 16:4). They may try to interpret their own dreams, but the Apostle Peter said that “we have the prophetic word more fully confirmed” (2 Pet 1:19a), so we don’t need to interpret anything because it can be confirmed by the Word of God. If you’re seeking “a word from God,” then know this: “Long ago, at many times and in many ways, God spoke to our fathers by the prophets, but in these last days he has spoken to us by his Son, whom he appointed the heir of all things, through whom also he created the world” (Heb 1:1-2). We already have “the faith that was once for all delivered to the saints” (Jude 1:3b). The Bible never teaches there are new truths coming and from new prophets. The tense in the Greek in all of these verses is past tense, meaning it has already been done. This also means there is no more new revelation coming because we already have the whole counsel of the whole Word of God. God speaks to us…but He does it by His Spirit, but most clearly in His Word. There’s no mystery to solve because “now he commands all people everywhere to repent” (Acts 17:30). It may not sound very mystical, but it is the truth.
When the gospel of Jesus Christ is written about, it’s often called the mystery because in former times, it was not known. Of course, salvation for the Old Testament saints was still available, but it was not as clear in the Old Testament as it is in the New Testament. God had hidden some of these things in the past, but now, God has made know to us the mystery of His will according to His good purpose (Eph 1:3-14), so “The secret things belong to the Lord our God, but the things that are revealed belong to us and to our children forever, that we may do all the words of this law” (Deut 29:29). If God has a hidden purpose or will, we have no business trying to find it out. If God wants us to know, He will make it crystal clear, so those things we cannot know for certain are things we have no business seeking out. For the believer, we are to do what we know is true in the Bible and not leave things up to us to figure out. God has made plain what is necessary for us to be saved, so pursuing secret knowledge is a waste of time. We know what God’s will is because it’s revealed in Scripture, so there is no need to try and find the hidden will of God or to search for mystical knowledge or seek after a mystical experience. 
One thing I’ve noticed is that mysticism tends to be an individual, subjective practice that depends on feelings and subjective thoughts, whereas biblical Christianity is both an individual relationship with Jesus Christ and one that’s lived out in community, or in the Body of Christ, the church, so “His divine power has granted to us all things that pertain to life and godliness, through the knowledge of him who called us to his own glory and excellence” (2 Pet 1:3). What God has plainly revealed we are to act upon, but what God has not plainly revealed in Scripture should be evaluated to see if it is from God…or from the other god of this world, since “the god of this world has blinded the minds of the unbelievers, to keep them from seeing the light of the gospel of the glory of Christ, who is the image of God” (2 Cor 4:4). The main things are the plain things, but the mysterious things should be left to God. Satan would love you to be seeking out the mystical experiences because this takes your eyes off of Christ, so why not rather focus on the clear and not the cloudy, because who knows the source behind these mystical experiences?
Here is some related reading for you: Christian Dream Interpretation. Does God Use Dreams To Speak to Us? 
Resource – Scripture quotations are from The Holy Bible, English Standard Version® (ESV®), Crossway Bibles. (2007). ESV: Study Bible: English standard version. Wheaton, Ill: Crossway Bibles. Used by permission. All rights reserved.