Is Christian mysticism biblical or is it ok to do? I am thinking of practices such as lectio divina and the theosis, for example.
This is an interesting question. Actually, let me re-word your question if you do not mind. Let me ask two questions.
1. Are some or all of the practices of the important Christian mystics biblical in the sense that the Bible tells us we ought to do these things?
2. Are some or all of the practices of the important Christian mystics allowable for a Christian to do (like you say… OK), and might they be helpful to improve the spirituality of disciples of Jesus?
The answer to the first question is mostly no. It is rather dubious to claim that the practices of John of Damascus or Theresa of Avila or John of the Cross or the many other famous Christian mystics are “biblical.” With some notable exceptions, I do not know of New Testament passages which prescribe the kinds of spiritual exercises which are described by the Christian mystics. If I am restricted to this first question of whether these practices of the Christian mystics are “biblical,” then I will say that Christian mysticism is mostly not biblical. It is not commanded in the Bible that we take part in most of things the mystics do, such as the lectio divina.
However, although I believe this is a relevant question, it is a good idea to go beyond this question. To illustrate this, is it “biblical” to drive to church in a car? The answer is no! It is not biblical. Nowhere in the Bible are we commanded to go to church in cars, and nowhere is it even allowed for or even suggested! However, driving to church is a great idea, especially if you live several miles from where the church meets. We can apply this rather silly example to more important questions about Christian mysticism.
So, let me pass along to the second question. Is it a good idea for disciples of Jesus to take on some of the proposed practices that Christian mystics have described? The question is not whether lectio divina is biblical, but whether it is helpful for Christians to do it. There are many other practices which the practitioners or Christian mystics have advised, such as Christian meditation techniques, fasting, retreats, silence, sabbath-keeping and many more. Actually, some of these are biblical, such as fasting and meditating on Scripture, but some are not.
My answer is that God wants us to know him, to have an intimate dependent relationship with him. God wants us to deeply understand Scripture and to have a sense of the Holy Spirit working in our lives. Any of these activities of the Christian mystics, if they are helpful toward these ends, then they are a good idea. My experience is that people who devote themselves to a range of these spiritual disciplines generally become closer to God over time. If so, then good! Is it possible to go over-board, relying on spiritual disciplines so much that it becomes religiosity? Possibly. Might we be so “busy” doing mystical spiritual exercises that we would draw back from important or even commanded practical Christian works? Possibly. Both are possible extremes to which we should not go, and some have done so. However, the implied question you are asking is whether it is a good thing for Christians to take on some of these so-called spiritual disciplines, even if most of them are not directly mentioned in Scripture. My answer is a solid yes! Yes, that is, if they are helpful toward achieving the end of becoming more Christ-like and having a deeper and more fruitful relationship with God, and do not distract us from some of the practical things of regular Christian living. I propose that for most of us, we should do more, not less in terms of spiritual disciplines, even if the mystics are part of Christian groups with which we are not aligned.
Christian mysticism and spiritual exercises are a bit of a fad right now in many of our churches. To me, this is mostly a good thing. It is a good fad! However, it is possible that some might go so strongly in this direction that they may become distracted from other of the Christian basics. This a possibility for us to watch out for, but, overall, I say without reservation that the renewed attention to spiritual disciplines is a good, not a bad thing.
To summarize, not “biblical,” but in most cases a good idea.